The Reagan administration's spending and tax cuts will significantly alter income distribution in the United States, with the poorest four-fifths of families receiving a smaller share and the richest one-fifth getting a bigger share than before the administration's changes, according to a study in this week's National Journal.

The National Journal, an independent news magazine, said it used census, Congressional Budget Office and congressional committee statistics to calculate the anticipated impact on cash income of the Reagan spending and tax cuts that will be in effect in 1983, using 1981 national income statistics as a base.

The National Journal's calculations showed that families in the top 20 percent of the income scale will end up with higher average cash income and a larger share of the total national income than before the Reagan proposals went into effect.

But the families at the bottom one-fifth of the income scale would end up with lower average incomes and a smaller share of the total national income than before the Reagan changes, and this does not take into account reductions in food stamps and other in-kind benefits that go mostly to the poorer groups.

The families in the top one-fifth would gain $36 billion in cash income whereas the bottom one-fifth would lose $1.2 billion.

The National Journal said that while every group will benefit somewhat from tax cuts, the impact of the tax reductions for the poorest group won't offset benefit cuts. For the richest families, they will more than offset them. For example:

* Families in the bottom one-fifth, the National Journal said, averaged $3,820 in cash income after federal taxes in 1981. Assuming the same basic level of income in 1983 for calculation purposes, the National Journal said the average family in this poorest one-fifth of the population would have an income of $3,760, or $60 less. In addition, the National Journal calculated, this group's share of total national income, already 4.4 percent, would drop to 4.2 percent.

* Families in the second poorest one-fifth averaged $9,430 in cash income after federal taxes and got 10.8 percent of the total national income in 1981, the National Journal said. The Reagan changes would raise their income a bit, to $9,450, but their share of the national income would drop to 10.4 percent.

* The third segment of the population would go up slightly in net after-tax income, from $15,090 to $15,370, but its share of the national income would drop from 17.2 percent to 17 percent.

* The fourth segment would also go up, from $22,030 to $22,680, but its share would drop from 25.2 percent to 25.1 percent.

* The richest group, the top fifth, would go up the most, from $37,170 for the average family to $39,200, and its share of national income also would rise, from from 42.5 percent to 43.3 percent, according to the National Journal's calculations.