Measurement device independent quantum key distribution is immune from all the detection attacks, so the final key is unconditional secure combined with the decoy state method. This paper analyzes statistical fluctuation analysis of finite key for the yield of single photon and quantum bit error rate of measurementdeviceindependent quantum key distribution by adopting the law of large number, and simulates the yield of single photon and the key rate by changing the key (N=106~1012). The result indicates that the secure transmission distance in optical fibre transmission will decrease to 260 km and 75 km respectively with the reduction of the key length, and the secure transmission distance comes to 295 km , which is close to the limitation when the key reaches to N=1012.