Abstract: By use of modified FELOScode,3D-shortpulse-simulations of waveguide FEL oscillator are done to testify the feasibility of FIR FEL under consideration. The result of 25% small signal gain is obtained. The time jitter of +lOps comparing 50ps duration of the electron micropulse is permissible for lasing which degrades small signal gain and laser power little.