Republican presidential candidate Marion G. (Pat) Robertson disclosed yesterday that he earned $202,000 in income last year, mostly from book royalties and his post as president of the Christian Broadcasting Network.

The candidate paid about 11 percent in federal taxes last year because charitable contributions to CBN and losses in a horse breeding business lowered his taxable income.

In 1985, according to copies of portions of his last two federal income tax returns released yesterday, Robertson and his wife Adelia reported a total income of $132,627 and paid about 9 percent in federal taxes.

His book royalties were put then at $76,525, far below the $185,372 he reported in royalties last year. His horse breeding loss in 1985 was reported at $31,883; the loss jumped to $56,049 last year.

Yesterday was the first time Robertson, who founded and ran the $180 million a year tax-exempt network, has provided such detail on his finances. He is one of the first 1988 presidential candidates to do so this year.

The returns were described as "actual copies" of the Robertsons' joint returns. The release did not include backup tax schedules.

Constance Snapp, communications director for Americans for Robertson, the campaign committee, said at a news conference that she did not release the full returns because she didn't think reporters would be interested. When pressed, she said she would release the tax schedules next week.

Snapp said with a smile that Robertson had told her he would offer to sell his horse breeding business for $25,000. Robertson, who listed his occupation as "broadcaster/author/farmer," raises the horses on a $400,000 estate that has been provided to his family rent-free by CBN.

Robertson's relationship with CBN has been an issue throughout the 15 months since he formed a committee to "test the waters" for a possible presidential bid because of reports that he might have used the resources of the tax-exempt network to further his political ambitions.

Snapp said yesterday that since Robertson declared formally as a candidate and severed his ties with CBN on Oct. 1, he has paid $2,000 a month rent -- determined by the network's accountant -- on the residence and will probably move.

She said she didn't believe Robertson earns royalties on the large quantities of his books CBN buys from his publisher to give to donors to the network.

The news conference here was dominated by questions about a Washington Post article yesterday about the sale of the campaign committee's computer system to a Denver firm, which resulted in a $337,500 receipt for the campaign on Sept. 30. The report said the sale had been made to a shell company that had been set up by a lawyer who was one of Robertson's campaign aides and appeared to be for at least $100,000 more than the system cost.

The article said the deal raised questions about whether the unknown purchasers might have made an illegal contribution to the campaign. It noted that federal election laws generally prohibit a committee from selling assets during a campaign for more than the $1,000 contribution limits because it is considered a form of fund-raising.

Snapp was unable, at first, to explain many of the details of the transaction, including the identity of the people who made the $337,500 "deposit" on the sale or the final sales price.

She insisted that the transaction was a proper business arrangement and said the price was not inflated because, though the sale wasn't final, it might include other computer equipment the campaign has bought since the initial purchase of the system for $233,480 last January.

Snapp said the transaction was not an attempt to raise money for the campaign, which has reported to the Federal Election Commission raising and spending more than $11 million.

She said the campaign was selling the computer system so it could buy a better one.

Snapp said after the news conference that she had not returned calls from The Washington Post about the computer sale because the campaign has been disappointed in The Post's coverage of Robertson.

After checking with headquarters, she furnished some additional detail on the computer transaction to three remaining reporters.

Snapp said she wasn't sure whether the Denver firm was picked to arrange the purchase because one of its members, Clarence A. Decker, was a Robertson campaign aide. But she said the firm specializes in such sales.

She said she still could not identify the members of what she said was a limited partnership that was buying the equipment or whether they were Robertson supporters. The final sale price and the exact equipment sold has not been determined and the transaction will not be final until an appraiser places a value on it, she added.