Abstract: In order to study the diameter, length and location variations of the diameter varied pipe affected to the flame propagation velocity of the gas explosion, gas explosion tests were conducted in the horizontal pipeline of the equivalent diameter horizontal pipeline and after the different specification diameter varied pipe add ed. The test results showed that in the equivalent diameter horizontal pipeline, the flame propagation velocity would be decreased with the flame propagation distance i ncreased. After the diameter varied pipe added, the flame propagation velocity would be slightly increased at the initial stage and then highly reduced. The flame propa gation velocity would be increased with the diameter of the diameter varied pipe increased and would be reduced with the length of the diameter varied pipe increased. When the distance of the diameter varied pipe in the horizontal pipeline was more closed to the flame source, flame propagation velocity would be highly reduced with t he location distance more closed to the flame sources. Diameter varied pipe would cause the changes of the flame propagation velocity in tendency and to possible eli minate such case occurred could reduce the danger degree of the gas explosion.