Using Spalart-Allmaras turbulence model, local head loss of sudden expansion in pipe was simulated when Re was less than 5500. The results showed that the simulated piezometric head agreed well with the test. The simulated coefficient of local head loss was lower than actual value when velocity head was calculated with the average velocity of pipe section and the kinetic energy revise factor was 1.0. The influence of kinetic energy computation method was greater than that of the friction head loss occurred at the sudden expansion pipe on local head loss.