Against the design scheme of 8 m ring solar telescope (8m-RST) of Chinese Giant Solar Telescope (CGST), the model of the edge sensor and optical sensor response in segments displacement is established, and the influence of the segments in-plane displacement on the surface shape of ring segmented primary mirror is analyzed. Considering the segments' displacement introduced by zenith angle change and temperature change for telescope truss, it's found that the displacement will destroy the effect of surface shape keeping of 8m-RST. To avoid the adverse effect of the in-plane displacement on 8m-RST, an improvement plan of the corresponding control model for the primary mirror is proposed. Which is correcting the set value of the edge sensor reading and the optical sensor reading, by adding two groups of gap sensor and one group of shear sensor among the edge of neighbor segments and using increased detection amount to estimate the in-plane displacement. When the detection accuracy of the gap and shear is less than 23 nm, the improved plan can meet the surface shape requirement of 8m-RST scheme for CGST.