Analytic expression with the effect of phase shift on reflection for full width of maximum height of a typical MEMS-based FP cavity is deduced by transfer matrix method. The relationship between full width of maximum height and interference order is simulated with the analytic expression,so does the relationship between full width of maximum height and the gap of the two reflectors. And the result is compared with full width of maximum height without considering phase shift on reflection. The effects on full width of maximum height caused by phase shift on reflection and reflection index respectively are discussed too.