Causes affecting behaviors of the optical add/drop multiplexer (OADM) based on Mach Zehnder interferometer and fiber Bragg gratings have been discussed. Based on theoretical analysis and simulation, a method of analyzing and adjusting the parameter of the OADM by monitoring the back reflected spectrum at the input port and by local heating has been proposed. It can also be used to simulate the process of trimming by UV exposure. Experimental results have proved the feasibility of this method.