King Fahd of Saudi Arabia, seeking to eliminate official corruption and guard against fundamentalist Moslem agitation, plans to ease the monarchy's tight control over Saudi affairs and broaden participation in government decisions, according to sources close to the royal family.

As described by these sources, Fahd tentatively is preparing to promulgate soon a detailed set of rules, derived from Islamic law, that will serve as a basic or supreme law for Saudi Arabia.

A major aim of these rules, the sources stressed, would be to counter the widespread image of many Saudi princes and officials as playboys who get huge commissions on government contracts and squander vast sums on high living. They quoted the king as saying he is determined to crack down hard on wealthy Saudis who "shower dancers and singers with bank notes in European nightclubs."

The sources said that to enforce the rules and broaden the base of government, Fahd plans to appoint a consultative commission, drawn from different sectors in Saudi life, that theoretically will have a powerful voice in shaping the kingdom's internal and foreign policies. In particular, the sources said, the commission will help Fahd apply the new laws, with emphasis on eliminating corruption, abuses of power and bureaucratic red tape.

Such steps would mean significant changes in the almost feudal governmental system that traditionally has kept all decision-making power in the hands of the large royal family and a few trusted ministers.

The information about these purported changes has been made available in the United States during the past few days to a small group of officials and television and print journalists dealing with Middle East affairs.

The sources who supplied this information are identified with factions in Saudi Arabia anxious to promote reform and modernization. Since most decisions of the royal family traditionally are kept secret or discussed publicly only in a cryptic manner subject to varying interpretations, it is not clear whether the information is an accurate reflection of Fahd's thinking or an attempt by some Saudi power centers to influence the course of events there.

However, independent experts on Saudi Arabia have confirmed that the information is consistent with what they know about Fahd's intentions and the planning he has set in motion. Less clear, these independent sources cautioned, is how imminent any changes will be and how much representation and power actually would be given to any consultative commission.

That Fahd is contemplating changes of this magnitude is described by all the sources as a sign of his determination to protect his dynasty from the kind of internal unrest that led to the overthrow of the shah of Iran by Moslem extremists.

These sources said Fahd, who became king in June after the death of his brother King Khalid, has been especially disturbed by a surge of anti-monarchy agitation touched off within his country by agents of Iran and the radical Arab regime in Syria. The sources added that he is particularly mindful of the connection between internal dissatisfaction and a sense that much of his nation's vast oil wealth has been squandered by the ruling elite through corruption and ostentatious consumption.

The sources quoted the king as having recently told a group of advisers:

"I have heard that some wealthy travelers shower dancers and singers with bank notes in European nightclubs. We cannot allow this to happen. Money is a weapon which must be properly used; otherwise it can cause harm.

"This type of conduct is a disease that needs to be cured. I am thinking of attaching a set of rules to Saudi passports to remind travelers that they belong to a Moslem country which has its traditions, its positions and its reputation. It is their duty to preserve them. And we shall not be content with issuing these rules of conduct. We shall establish some form of control."

The sources said that the new law being contemplated by Fahd cannot be compared to a constitution in the western sense. But it still would represent a major departure from the traditional Saudi system because it would be designated clearly as the supreme law of the country and thus would be binding on everyone, including the royal family.

Planning of the reported changes began a year ago under the direction of Fahd's brother, Prince Naif, who is interior minister. The sources said Naif was charged by Fahd to extract from the Koran a set of about 200 rules based on the Islamic principle of "shura" (roughly meaning "consultation") that will serve as a channel for airing grievances and broadening participation in govermental decisions.

However, as some sources noted, such shifts toward giving people a greater voice have proven tricky in the traditionalist societies of the Persian Gulf. Two small neighboring states, Kuwait and Bahrain, recently disbanded experiments with parliaments because they came into conflict with the ruling families. But two others, the United Arab Emirates and Oman, have consultative councils of the type being considered for Saudia Arabia.