In order to improve performance of sampling optical signals as well as level of integration for Triplexer monitor, a wavelength-insensitive directional couple (WIDC) based on asymmetric-taper was presented. The WIDC adopted SiO2-on-Si buried type optical waveguide structure, with the taper waveguides constituting the coupling region at the center part and the S type bent waveguides used for the signal light into and output. The transfer matrix method was used to analyze the normalization of the coupling optical power at the light monitoring output port. The center region of WIDC was regarded as asymmetric rectangular waveguides and the lateral region was regarded as symmetric rectangular waveguides. Numerical simulations by both the Finite Difference Beam Propagation Method (FDBPM) and Matlab showed that when the input wavelengths varied from 1 300 nm to 1 600 nm, the normalized coupled optical power achieved by the WIDC was 4%～8%. In particular, the splitting ratios of WIDC were 92:8, 96:4 and 93:7 respectively when the input wavelengths were 1 310 nm, 1 490 nm and 1 550 nm correspondingly with the polarization variations less than 5% under the condition that maximum width and minimum width of the tapered waveguide were 5.50 μm and 3.35 μm respectively. The devised WIDC offers many advantages such as small size and high reliability and suitable for the Planar Lightwave Circuit (PLC)-based Triplexer.