This paper presents test reliablity of Uster AFIS for impurity test by calculating the Reliable Test Time (hereinafter referred to as RTT) and CV% of test results. The CV% value of test results of impurity-related parameters in card sliver obtained in 8 experiments totally with 313 different plans were calculated. By statistical analysis method, the reliable test time of AFIS for some impurity-related parameters was estimated. It is concluded that the impurity result obtained by 10-time tests with AFIS was inaccurate. The reasons for this are that the sample weight is too small, the impurity is unevenly distributed and the impurity in card sliver may have some loss in the manually-sampling process.