A probability-based two-dimensional (2D) alerting algorithm in independent parallel instrument approach procedure is studied.In view of the problems that there exist fixed modes,extra and delayed alarms in traditional algorithm for independent parallel instrument approach,which may bring poor alerting performance and reduce flight efficiency,a new 2D alerting algorithm is proposed based on probability.This algorithm uses Monte Carlo simulation to obtain the collision probability and the alerting region,in which the collision probability equals to the danger level,to define the alerting condition in combination of the collision curve.Taking a certain airport as the flight environment,different locations of aircraft are selected to verify this algorithm.The results demonstrate that the improved algorithm can effectively eliminate extra and delayed alarms, which can provide a reference for further optimization of the alerting algorithm.