Based on formamidinium lead trihalide perovskites have emerged as promising photovoltaic materials attributed to their superior absorption coefficient properties. However, one big challenge is the material phase stability and thermal stability at high temperature. In this work, a large quantity of rubidium (Rb) ions is incorporated into formamidinium (FA) perovskite thin films to improve the material phase stability and thermal stability. Photodiodes based on the optimized FA0.7Rb0.3PbI3 perovskite deliver a high responsivity of 0.43 A W-1, detectivity of >1012 Jones, relatively large linear dynamic range of 125 dB, and an ultrafast response speed of approximately 300 ns. Moreover, these photodiodes present lower dark current and higher photocurrent after baking at high temperature. These results are very promising for photodetection at high operational temperature. In addition, the high ratio rubidium incorporated perovskite films may have great potential in fabricating other high-performance opto-electronic devices, i.g., light-emitting diodes and solar cells with excellent phase stability and high temperature thermo-stability.