Currently the FY-4A/GIIRS data assimilation directly uses the cloud detection product from Advanced Geostationary Radiometric Imager （AGRI）， and the whole channel data has to be removed when the corresponding IFOV is contaminated by cloud， leading to the loss of some available channel information. In order to improve the utilization rate of those data， a cloud detection algorithm is set up with both GIIRS observation and RTTOV simulation by adopting the method given by McNally， together with radiation characteristics such as GIIRS sensitivity. The results of the proposed method are generally identical to those from the AGRI CLMs， where some minor differences will occur for IFOVs with a certain of fractus. Using the derived the heights of cloud top for each IFOV， the clear channels with respect to the cloud IFOVs can be identified with the data utilization increased by around 13.76% in statistics. This proposed cloud detection algorithm can provide an important reference for GIIRS data assimilation.