To achieve high polarization measurement accuracy of a particulate observing scanning polarimeter (POSP), onboard polarization calibrators are designed and a novel measurement method for their polarization state value is studied. First, the instrument overview, measurement principle, and polarization calibration model of the POSP are introduced. Second, based on the instrument characteristics, we proposed earthshine combined linear polarization calibrator (LPC) or non-polarization calibrator (NPC), two types of standard polarization state sources, and then design the LPC and NPC in detail. Finally, Marius's law is used to determine the polarization state value of the LPC. Data from observations of the satellite nadir are utilized to choose a underlying surface with a degree of linear polarization less than 0.4 as the effective light source for NPC. The two calibrators meet the requirements of full path, full aperture, and end-to-end calibration, ensuring the long-term polarization measurement accuracy of 0.005.