In order to realize quick response and effective detection in emergencies such as flight missing and natural disaster, an optimize-by-priority on-orbit task real-time planning algorithm based on priorities of targets was proposed. First, ground targets were divided into several priorities in order of importance, and the task planning was transformed to an optimization problem with objective of maximizing the imaging gain by analyzing constraints between target locations and satellite performance. Then, the targets of the same priority were selected by the rule of “first come, first served”, lower-priority targets were inserted into current optimal imaging sequences filled with several former-priority targets, and the maximum imaging gain will be obtained. Finally, a 500 s planning simulation of 18 ground targets was given. Results indicate that the algorithm proposed can give imaging sequence with the maximum gain within 1 s. The proposed algorithm calculates faster and gives more stable results as compared with the genetic algorithm.