The method of calculating the flame front speed of the explosion in the pipe is researched by theoretical analysis and experiments. The model of calculating turbulent burning velocity and expansile velocity by pressure is deduced after the simplification of flame front speed to be the result of turbulent burning velocity and expansile velocity, and the method of calculating flame front speed is advanced by pressure, laminar burning velocity, turbulent burning velocity and expansile velocity. The calculating analysis and explosion experiments of the premixed mixture under the condition of the lean fuel (equivalence ratio equal to 0.967) and rich fuel (equivalence ratio equal to 1.21 and 1.45) indict that the pressure is linear with the accretion of length to diameter ratio on the longitudinal direction, and the laminar burning velocity is linear with the pressure variation, and the change rate of laminar burning velocity is larger when the equivalence ratio approaches the stoichiometric one while the laminar burning velocity is the largest at the equivalence ratio of 1.21. The effect of pressure variation and laminar burning velocity increasing on turbulent burning velocity is not apparent, but is apparent on the expansile velocity, so the pressure and laminar burning velocity is the decisive factor in expansile velocity but not in turbulent burning velocity.