The monitoring of weld pool shape is very significant for welding quality control in the process of all-position pipeline welding. A set of molten pool monitoring experiment device was designed to collect clear molten pool images. The contours of molten pool was detected by using image processing technology. The shape of molten pool was redefined by using the length, the width and the head contour binomial coefficient of the molten pool. Orthogonal experiment and range analysis were used to study the influence of welding current, arc voltage and welding torch swing speed on the shape of the molten pool.The results show that good weld forming can be obtained by using the optimized welding parameters combination.