It exits large computational error that the computational formula derived from the untrucated poppet valve is adopted for the cross section area of the truncated popper valve. The flow pattern of the truncated poppet valve was studied with CFD visual algorithm. The position of cross section was confirmed based on the flow filed characteristics and the concept of the turn opening position was proposed. It was concluded that the cross section area of valve had a key turning point when the stroke of truncated poppet valve was large. The established technique and computational formula of the cross section was differed from the stroke and the converging/diverging flow. The result of the cross section area and the flow rate was more exactitude by adopting the new computational formula for the truncated poppet valve than by adopting the traditional formula. Comparing the flow filed characteristics of diverging flow with that of the converging flow, it was pointed out that the channel flow in the truncated poppet valve should consider the throttling pressure loss and the local pressure loss. It was summarized that the difference of the flow filed characteristics was induced by the local pressure loss.