Applying discrete optical component technology, a novel Reconfigurable Optical Add/Drop Multiplexer (ROADM) based on the four-fiber collimator and thin film filter is designed. Four ports, including input port, drop port, add port and output port, are arranged on the same side of ROADM. Simulations to the designed ROADM, as well as bit error rate and eye diagram in networking, are performed using Optisystem. Simulation results show that the designed ROADM has loss of 2 dB and balanced channel gain. In networking, the bit error rate is less than 10-9 and Q value is more than 6 with clear eye diagrams at speed of 2.5 Gb/s and distance of 75 km, which verifies that the designed ROADM is suitable for the metropolitan area network system.