According to the 3 Dimension model of the side member frame, a finite element analysis (FEA) model of semi-trailer tractor frame was established and the approach of applying loads and constraints was discussed. Then the frame stress was analyzed under four working conditions as well as the eigen frequency was calculated too. It was found that the maximum stress appeared on the rear channel side-rail under the bending and twisting condition, however it still meets the strength requirements. Natural vibration frequency is far away from the exciting force's frequency such as the suspension vibration and the engine incentive. The harmonic analysis results shows that the significant stress appeared on the connection part of the channel side - rail and the second cross beam, the balance beam axis, tubular cross member.