The necessity of using lightweight primary mirror in spatial optical remote sensor is statal in this paper. The suffoce figure of primary mirrors in shapes of double-concave,plain-rear, single-arch and double-areh is computal, Which works out a conclusion that double-concave mirror is the best one, supported in three-point pattern The regularity of wave front error (RMS value) vs. normalized radius r / R is studied, which indicates that the best support radius is 0.74