Abstract: To address the problems of heavy computation and low efficiency in traditional Monte Carlo-based logging-while-drilling (LWD) gamma simulation, which restrict the application of gamma logging in real-time geosteering, a fast forward modeling method for LWD gamma is developed in this paper. Based on the working principle of gamma logging, this method analyzes the contribution rate of the spatial position of media to the measurement response of gamma detectors and establishes the flux sensitivity function of gamma logging. The gamma response can be rapidly obtained by integrating the discrete values of the gamma sensitivity function and the radioactivity intensity of corresponding spatial units. Compared with the traditional Monte Carlo method, its single-point calculation time is reduced from 10 minutes to less than 2 milliseconds, realizing real-time simulation with high computational efficiency. Meanwhile, it can take the influence of environmental factors on logging responses into account. Its relative error compared with the high-precision Monte Carlo method is within 5%, ensuring reliable calculation accuracy. The simulation method based on the flux sensitivity function provides a fast simulation approach for LWD gamma in stratified strata and offers an efficient data processing technology for rapid model comparison and matching in geosteering.