Premier Aldofo Suarez has won a vote of confidence in Parliament after a major Cabinet reshuffle and a policy declaration viewed by commentators as a watershed in his effort to strengthen the democratic system in Spain.

In theory, Suarez and his moderate Union of the Democratic Center Party seem set to continue in power until 1983, when elections are due. In the vote last night, he received the support of two minor regional parties.

As solid as Suarez' position might appear at first glance, however, the compromises he made to win essential support from regional parties and the splintering of his power within his own party as indicated by new Cabinet choices underscore a fundamental fragility in his position.

That instability was underlined when the Basque legislators, the key to the most serious regional problem in Spain, joined forces with a right-wing conservative party and aligned with the Socialists and Communists to vote against Suarez.

The instability had been illustrated last week when Suarcz dropped eight ministers and formed his fifth Cabinet in four years. The backdrop to the reshuffle was an economic record that has led to an unemployment rate of nearly 12 percent -- the highest in Western Europe.

In general elections 18 months ago Suarez's party gained 168 seats, 10 short of an absolute majority in the 350-member Congress of Deputies, as the lower house of parliament is called. Since March 1979 the government has seen its power eroded in regional parliamentary elections and it narrowly survived a censure motion submitted by the main opposition party, the Socialists, in May. The issues were the all-important state of the economy and the program for regional administration to end the centralization under Francisco Franco.

Suarez, in an attempt to reassert his authority in the often passionate debate that opened Tuesday, comented pacts with the northeastern Catalan regionalist party and with a nominally left-wing Andalusian socialist party representing the underdeveloped southern belt of Spain.

The pacts enabled Suarez to win the confidence vote 180 to 164 but raised questions about how long the alliance might last.

For both the Catalans and the Andalusians, the trade-off was a promise by Suarez to speed up regional rule at the expense of the power of Madrid.

The bid for regionalist backing was marred by the decision of the Basque Nationalist Party, the majority party in the Basque country, to vote against the government. The Basque party, staunchly nationalist but ideologically centrist, had boycotted the Congress since January in protest against the government's regional policies.

By returning to parliament, the Basque group raised hopes that it would would back the government. But, in an ominous indication of confrontational politics in the northern provinces, the Basque dismissed the government's offerings as lacking in credibility.

More than 70 people have died this year in political violence in the Basque country, where seccesion is a major political issue.

Suarez declined to give specific economic details in his policy projection but announced an overall aim to boost the present near-zero growth rate to between 4 and 5 percent per year by 1983. His main tool would be activation of public investment.

Added promises to cut public spending, widen indirect taxation and liberalize interest rates were viewed with suspicion both by the left -- who disputed the government's inability to carry out the measures -- and by the right, who saw them as a recipe for inflation.

Attacked on both fronts, Suarez relied on his new government team to deflect the questioning during the debate. The team, known here as the "barons," includes the cofounders with Suarez of the Union of the Democratic Center at the time of Spain's first post-Franco general elections in 1977. They have now been brought back into the Cabinet to shore up the premier in what political commentators have termed "Suarez's last chance."

Among the party stalwarts brought back to power in the Cabinet reshuffle was a prominent critic of Suarez, former treasury minister Francisco Fernandez Ordonez, who is credited with introducing a progressive income tax in 1977. Later dropped from the Cabinet, he has publicly sniped at the premier for his alleged conservatism.

Fernandez Ordonez, now justice minister, will oversee introduction of a divorce law and the resolution of mixups between military and civil jurisdictions -- a hangover from the Franco years.

The return of the "barons" is interpreted as Suarez's recognition that he needs their experience and their hold over the party rank and file.

In contrast to previous reshuffles, the Cabinet changes reflected a waning of the premier's grip as he was forced by adverse circumstances to bring back politicians he had formerly hoped he could dispense with.

The consensus among Spanish political observers was, however, that Suarez had at least earned a breathing space -- important in the often giddy Spanish political scene.