In photovoltaic systems, the installation tilt angle of modules plays a critical role in determining both performance and economic returns. Existing methods for calculating the optimal tilt angle typically rely on complex solar radiation and anisotropic sky scattering models, which, despite their accuracy, involve intensive computational efforts. Based on the relationships between the proportion of direct radiation, latitude, and the optimal monthly tilt angle, this study proposed a method to calculate the monthly optimal tilt angle. Taking Zhangye City as an example, the accuracy and reliability of the proposed method were verified in comparison with the traditional method. Using this method, the monthly optimal tilt angles were calculated for three cities in China located at different latitudes, to reveal the annual variation patterns of these angles. The results showed that, at the same proportion of direct radiation, the monthly optimal tilt angle increases with latitude. Over the course of a year, the monthly optimal tilt angle first decreases and then increases. In low latitude cites, the monthly optimal tilt angle may even become negative.