For the next generation of Chinese geostationary meteorological satellites (FY-4) an optical lightning mapping sensor is planned to observe lightning on a real-time, continual basis. The measurement will detect the radiance discharged by lightning and transferred up to the cloud top at a near-infrared band. One important and urgent attention of the pre-study is how the instrument observation geometry quantitatively impact the received signals. A Monte Carlo approach is applied for simulating the transfer of lightning and the lightning radiation signatures which will be obtained by FY-4 lightning mapping sensor. The study focuses on the quantitative relationships between the observed lightning radiance and several key observation geometry parameters, such as satellite observation angle, pixel size and the horizontal location of lightning in the pixel. This will provide extremely valuable informations for the future application of lightning data observed by FY-4 satellite.