We present the design of a newly conceived triplexing optical microsystem that can provide bases for three wavelengths splitting function for gigabit passive optical network application. Three wavelengths consist of one for the upstream, 1310nm wavelength signal and the other two for downstream, 1490nm and 1550nm wavelength signal. The triplexer module is designed in the form of a planar optical integrated circuit using three directional couplers. In order to increase wavelength splitting performance we arrange the directional coupler by cascaded connecting method and use the asymmetric Y-branch. We analyzed its performance characteristics by BPM. The optical crosstalk of the 1310nm signal to the 1490nm signal and the 1550nm signal are -27.92dB and -66.83dB, respectively in the upstream. In the case of downstream the optical loss is less than 0.5dB with 20nm bandwidth and the optical crosstalk reaching less than -30dB in the downstream is 19nm.