Abstract: According to the real conditions for SG-1 FEL, sideband instabilities are numerically simulated. Using WAGFEL code, the slippage due to different velocities between optical pluse and electron pulse is considered, which makes electrons trapped in ponderomotive well couple with each other at changeable phases in sychrotron oscillation, hence leading to the growing of sideband instabilities from intensity modulation of laser. This code is checked by comparison with the results of GINGER code of LLNL.