Abstract: The data obtained from astronomical satellite is telemetered to ground and converted into high products (such as energy spectra and light curves) for further analysis. In one of these steps, data preprocessing is a very important step. This work will take the Media Energy X-ray Telescope (ME) on the Hard X-ray Modulation Telescope satellite and the Gamma-ray Burst Polarimeter-POLAR on Tiangong-2 (TG-2) as examples to introduce the method of data preprocessing. In this work, we present a standard data flow for astronomical satellite, and also introduce how to use configure file to decode data and how to calculate the local time of the payload and GPS time. This is helpful to establish data preprocessing system for other astronomical satellites.