The traditional WDM network uses fixed grid, which can not meet the network requirements. An improved graph coloring based routing spectrum allocation(GT-RSA) algorithm is proposed. Firstly, the first k shortest path algorithm is used to determine the candidate route set, and the candidate route graph is established according to each candidate route set. Then, the mixed integer linear programming model of graph coloring theory is used to select the route, and the optimal solution is obtained according to the maximum saturation priority algorithm of branch boundary. At the same time, the coloring partition allocation of spectrum is completed. Finally, partition first-last-fit(PAR-FLF) algorithm is used to allocate spectrum resources. The simulation results show that GT-RSA algorithm has lower spectrum resource consumption, lower blocking rate and higher average link continuity rate.