In order to measure the rice flag leaf angle rapidly and nondestructively, a portable system for measuring rice flag leaf angle based on Android smart phone was developed. The rear camera of the smartphone was used for flag leaf images acquisition, and the flag leaf angle was obtained through image preprocessing, lines detection, K-means clustering and vector method. The interfaces and function modules of system software were designed by using the Android programming technology, which realized the extraction algorithm of rice flag leaf angle using JNI and Android NKD to call OpenCV database in the Android platform. It can complete the operational flows of new test establishing, material information inputting, flag leaf images obtaining, angle value outputting as well as interface saving. 80 plants of rices in four varieties were tested to verify the system performance. The test results demonstrated that, comparing with the measurement results of protractors, the average absolute error of this system was 1.34°, the relative error was 2.7%, and the correlation coefficient between the observed values and actual values was 0.997. It was indicated that the proposed system could effectively measure the rice flag leaf angles.