HPL is a Linpack benchmark package widely used in performance test of massively parallel systems. A law to determine block size NB theoretically, which breaks through the dependence on trial-and-error experiments, is found based on algorithm analysis and real test of HPL. According to that law, estimated execution time can be obtained through algorithm complexity. An emulation model is constructed to estimate the HPL execution time in more detail. Verified by real system test, the model is used to forecast the benefits of Linpack test brought by the intending system improvement. It is expected to afford reference to system improvement on optimizing HPL test in the future.