There are 108 beam position monitors in the PLS for the orbit measurement and closed orbit correction. As the BPM pickup electrodes are assembled directly on the 10m-long vacuum chamber, calibration of the electrical center of BPMs have been done with the external rf transmission-reflection method. Since the closed orbit distortion is still larger than 500 mm rms, we are developing the beam-based calibration method for the BPMs in PLS to improve the accuracy of the absolute position reading. In this report we present the method of beam-based calibration of BPMs and its experimental results.