As one of the most challenging problems in the QoS Routing and traffic engineering, the problem of delay constraints optimal path calculation has the non-polynomial(NP) complete complexity. The algorithm which is proposed in literature has some problem, such as not getting optimal path and being very hard complexity etc. So a delay constraints optimal path (DCOP) algorithm which can solve this problem well was proposed. By reduced the search region of the algorithm, the efficiency of algorithm can be improved effectively and an optimal loop-less path can be gotten. The self-adapting parameter design is adopted in this algorithm toimprove the adaptability on the network scale and the changing of complicated service. Using extensive simulations on random graphs and random assigned link weights, the huge improvement in complexity of the new algorithm is tested. The test also indicate that the algorithm has more adaptability and more practicability.