The 3 1/2 percent federal pay raise President Reagan has proposed won't mean much for rank-and-file white-collar workers, who stand to make about $20 more a week on average. But it will mean a $2,400 boost in the paycheck for top government executives and members of Congress when it goes into effect this January.

Under the current federal pay law, salaries of top government appointees, Cabinet members and those of senators and representatives will piggyback the general federal pay raise unless Congress takes action to deny itself, and perhaps other government executives, the raise.

Members of Congress now get $69,800 but that would rise to $72,200 next year under the pay plan. Cabinet officers who earn $80,100 would go to $82,900 next year and the salaries of other top government officials would rise accordingly.

Most of the 6,879 members of the government's Senior Executive Service would get pay raises of $2,000 or more in January, when their subordinates' pay goes up. The vast majority of the SES members--who are at six different pay levels--live and work in the Washington area.

Currently, pay for SES members ranges from $56,945 to $67,200. The bulk of the SES people are in the $61,000 to $63,000 pay range.

Under the proposal, pay for SES members would rise as follows:

Executive Schedule One would go to $58,938.

ES Two would rise to $61,292.

ES Three would be $63,646.

ES Four would be $66,000.

ES Five would be $66,800.

ES Six would be $69,800.

The Senate and House have tentatively approved a 4 percent raise to be effective in January. If that becomes law, the increase for most of the government's 1.4 million white-collar workers--from typists to the head of departments--would go one-half percent more than the president has recommended.

Congress may get the jitters about taking a pay raise in an election year. If that happens it could simply vote to exclude itself from the raise, or vote to deny it to all top executives.