Abstract: On the homemade planar-cryotarget shooting system, we have studied the effect of different filling rate on preparing planar cryotargets. The results show that with slower filling rate, the variation of hydrogen pressure in target is small. Furthermore, the liquefaction procedure is more observable and the corresponding duration of time is longer. Slower filling rate is helpful for the accurate measurement of saturated vapor pressure of liquid hydrogen at that temperature. A calibration method based on the saturated vapor pressure is developed to in-situ determine the temperature of liquid hydrogen isotopes in the target cell, and an optimum filling rate of 0.8 mL/min is obtained. These studies are important to provide high-quality and reliable hydrogen-isotopic planar cryotargets for ICF physical experiments.