This paper presents asynchronous blocking mechanism of transformer differential protection based on network sampling. The purpose is to improve the network adaptability of the transformer differential protection based on the background of smart substation of network sampling. It introduces the network sampling and its synchronism mode, as well as the reasons of asynchronous state. It studies the operation characteristic of steady-state differential transformers in the asynchronous situation from different manufacturers. Then the deviation angle of operation of transformer differential protection can be achieved according to the load change and different start values. Then the sampling asynchronous frames causing the mal-operation of differential protection are analyzed and solutions are presented. The transformer differential protection which uses the asynchronous blocking mechanism has been put into practice in the smart substation projects and it achieved good results in avoiding the protection mal-operation caused by asynchronous performance.