The Lyman-alpha Solar Telescope (LST) is one of the payloads for the Advanced Space-based Solar Observatory (ASO-S) mission, and it consists of a White light Solar Telescope (WST), a Solar Disk Imager (SDI), and a Solar Corona Imager (SCI). And one method of its in-flight flat field calibration is the so-called KLL method proposed by Kuhn, Lin, and Loranz in 1991. In order to study the influence of KLL method's time interval between two successive offset positions on the derived flat fields of the WST and SDI instruments, we use the full-disk images from the Helioseismic and Magnetic imager (HMI) and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) satellites to mimic the images of WST and SDI observations, and use them as test images for the KLL method calculation. Based on our calculation and analysis, we got the following conclusions: when we adopt the KLL method to obtain the flat feild, the shorter the time interval, the better the flat field accuracy. This puts some requirements for the spacecraft platform. Considering the time required for the satellite's attitude adjustment and stabilization as well as image acquisition, the time interval for SDI flat field needs to be within 240seconds. The WST flat field accuracy is not sensitive to the time interval between two successive offset positions. This conclusion also limits the time required for the satellite's attitude adjustment and stabilization.