Site dynamic balance is a important element in installation and maintenance of heavy rotary mechanism. It's hard to discharge testing weights, and the remanent weights will effect the result of balance test. The paper presents a kind of dynamic balance algorithm which need not to discharge the test weights, basing on influence coefficient. The principle of the algorithm is to balance the sum of original unbalance weights and testing weights by the result balance weights. In the algorithm , the influence coefficient of last time can be used optionally for the same rotor. Then developing a dynamic balance system building on it. The site experimental results indicates that this algorithm has a high balance precision, predigests the steps and reduces the operation time.