POLAR is a Compton polarimeter with 1600 pixels designed to measure the polarization of Gamma-Ray Bursts (GRBs). POLAR has been launched successfully onboard the Chinese space laboratory Tiangong-2 on 15th September 2016. The anisotropy of Compton scattering is used as the characteristic quantity to extract polarization degree and angle. The scattering angles are accumulated into a modulation curve and compared with simulated ones based on calibration data to determine the polarization. The differences of the detection efficiency in 1600 pixels between in-flight observation and simulation must be considered. Data from several bright GRBs are used for this purpose. We use the energy spectra of these GRBs provided by Fermi/GBM, KONUS/Wind and Swift to do simulations; the ground calibration data are used to characterize the performance of each individual pixel, including readout electronics. Comparing the simulation results with observation data, we obtain the detection efficiency of each of the 1600 pixels, which can then be corrected for measuring the polarization of these GRBs.