We studied numerically the short-pulse FEL oscillator system using properly defined bunching phase 0(B) and psi(B). In stable operation, we have found that the optical field "locks" the phase to pi/2 at the trailing edge, which gives the maximum gain. Moreover, electrons can be detrapped from ponderomotive bucket due to the spatial variation of the optical field. and this detrapping effect is a major cause of the limit cycle oscillation of the system. The 'bump' of the output power during the amplification usually exists at the near-perfect cavity synchronism regime. which can be explained as the change of the matching condition between electron micropulse and optical pulse. (C) 2000 Published by Elsevier Science B.V. All rights reserved.