Automated and intelligent optical microscopy necessitates the measurement and correction of the focal plane drift for many biomedical applications, such as long-term observation of living cells and pathological whole slide imaging. In this study, we designed a method to perform focal plane drift measurements based on the detection of the reflected spot position of a nonsymmetric beam. Through simulations using ZEMAX software, the shapes of the reflected spots were simulated on the sample surface at different defocus positions. Furthermore, an integrated focus plane drift measurement module was constructed and tested on a commercial upright microscope. The results reveal that the proposed module exhibits a drift measurement precision of 250 nm and a response time of less than 500 ms for a 60× oil immersion objective, satisfying the demand for long-term and high-resolution imaging.