Five U.S. senators announced yesterday that they are filing suit against the Senate in an effort to nullify the senatorial code of ethics approved April 1.

Led by Paul D. Laxalt (R-Nev.) the five said they particularly object to a code provision that limits to $8,625 the amount a senator may earn from speeches, articles and other outside activities in addition to his $57,500 salary. The provision goes ito effect Jan. 1, 1979.

They said the limit on outside honoraria and salaries adds a qualification for office that goes beyond what is contained in the Constitution. Laxalt also said that the provision limits the freedom of senators to have outside earnings and, in effect, deprives the voter of the range of choices he ought to have - by making it impossible for an individual to run for office if he needs to earn higher income than allowed.

The code of ethics was approved 86 to 9. All five senators who are filing the suit in District Court voted against it. Barry Goldwater (R-Ariz.), Carl T. Curtis (R-Neb.) S. I. Hayakawa (R-Calif.) and Lowell P. Welcker Jr. (R-Conn.) joined Laxalt in the suit.

Although their brief focused on the $8.625 outside honoraria-and-salary provision of the code, it aserted that the code's other provisions weren't legally severable and therefore the court was asked to declare the entire code "null and void" and to enjoin Secretary of the Senate J. S. Kimmitt and Chairman Adlai E. Stevenson (D-Ill.) of he Select Committee on Ethics from seeking to enforce any portion of it. Both Stevenson and Kimmitt said arrangements would be made to defend the Senate against the suit. "Of course the suit will be defended," Stevenson said.

If successful, the suit would knock out numerous other provisions, such as financial reporting requirements and restrictions on receipt of various gifts.

Curtis yesterday called the ethics code "a sham" that would "not do the job," and Laxalt said the idea of adding any qualifictions to office beyond those in the Constitution "would deny the right of the citizenry to have all others serve." Laxalt, Goldwater, Hayakawa and Weicker also said they will be in the Senate Jan. 1, 1979, when the limit goes into effect and it could be an unjust curb on their outside earning capacity.

Senate records show that in 1976 Laxalt earned $10,550 from outside speaking engagements, Goldwater earned $6,350, Curtis $7,528, Hayakawa $13,585 (he was not a senator then) and Weicker $7,500.

All except Curtis said they did not object to financial reporting requirements.

Hayakawa said, "The ethics code assumes the lecture honorarium to be a form of bribe, given by the sponsoring organization to influence the speaker . . . I am not capable of being bribed, and if I were, I would trust the people of California to vote me out of office."