Thanks, everyone.
I use formula:
//whilereadingrecords;
//numbervar c1; numbervar c2;
//if remainder(c1,3) = 0
//then (c1 := c1 + 1; c2 := c2 + 1;)
//else (c1 := c1 + 1; c2;)
if{DataTable_CHENJIANG.周期} <=3 then
1
else if ({DataTable_CHENJIANG.周期} > 3 and {DataTable_CHENJIANG.周期} <=6) then
2
else if ({DataTable_CHENJIANG.周期} > 6 and {DataTable_CHENJIANG.周期} <=9) then
3
..........
And it works...
I know it may be a little awkward，but it seems works now。