The schlieren photographs of specified test conditions (T=348 K,P=0.10-0.30 MPa,Φ=1.0,Φ_r=0%-8%) were selected to study the effects of dilution rate and initial pressure on laminar combustion flame propagation and the effect of measuring angle step on the accuracy of flame radius measurement. The evaluation indicators were then proposed to describe the distribution of flame radius for obtaining a more accurate flame radius value, which could provide more reliable data for the study of laminar combustion. The results show that the flame stability is worse and the radius extraction value fluctuates greatly in the undiluted condition. The flame stability and the fluctuation improve with the increase of dilution rate. The flame stability becomes worse and the flame propagation gets uneven when both the dilution rate and flame radius increase in the diluted condition.