Abstract: A numerical integration formula was introduced to calculate the response of planar HPGe detector to photons emitted from point source. Then the formula was used to correct the geometrical parameter of planar HPGe detector. ²⁴¹Am and ¹³⁷Cs point sources were placed at a certain distance (1-20 cm) away from entrance window to get the corresponding detection efficiency. The detection parameters were calculated in weighted least square fitting using the formula with the experimental efficiencies as formula results. This correction method was accurate and timesaving. The simulation result from MCNP using the corrected parameters shows that the relative deviations between simulation and experimental efficiencies are less than 1% for 59.5 and 661.6 keV photons with the distance of 1-20 cm.