The unevenly distributed traffic of Internet may lead to congestion and underutilization of network resources. Current methods of routing adjustment for load balancing often get a new path with excessively long length that can exacerbate the quality of service. This paper presents a new flow routing adjustment algorithm named LCBA (length-constrained most balanced algorithm), which can decrease the maximal bandwidth utilization rate of network with delay guaranteed. The experiment based on Abilene2 network topology and traffic data shows that LCBA can mitigate the congestion of backbone network effectively and the maximal bandwidth utilization rate can drease nearly 50% at most. This paper also evaluates the algorithm with an emulation method. Compared with current methods, the new algorithm can satisfy the demand of not only the length of key flow’s path but also the maximal bandwidth utilization rate. Moreover, the theoretical analysis proves that the computational complexity of LCBA is O(N2logN), which is better than that of a majority of methods nowaday.