To solve dithering problem when telescoping boom of a truck crane retracting,by measuring hydraulic dynamic data using instrument during retracing boom, the instant flow was found to drop to zero, which was the cause to the problem. A mathematic model was built based on boom and cylinder in practice. Friction force was found as the influence factor to flow at large scale. After replacing sliders with small friction coefficient, hydraulic dynamic data were measured again. The feasibility of the mathematic model and practical improvement were verified, and dithering problem was eliminated.