In order to forecast the subway traffic accurately and better the vehicle scheduling and site management，a short-term forecasting of subway traffic based on K-nearest neighbour pattern matching is proposed． By analyzing the subway traffic data，it is found that the day passenger flow development mode of subway has a certain law．Thus，an adaptive K acquisition algorithm based on the calculation of error rate is proposed，it can improve the uni-versality of the prediction algorithm by automatically obtaining the appropriate K． Finally，Guangzhou South Rail-way Station is taken as an example for the case study． The experimental results show that the method proposed is applicable to the subway passenger flow forecast of two different modes of traffic-holidays and non-holidays，its ave-rage prediction accuracy is about 90%． It can be seen that this method has a good application value．