Wedge shaped fiber (WSF) is the key component to acquire high coupling efficiency between input/output ports of planar lightwave circuit chips and fiber arrays. It is very important to analyze the mode spot and the mode field evolution of the lightwave propagating in WSF using numerical simulation tools. A three_dimensional equivalent rectangle approximation-staircase concatenation method (ERA-SCM) is proposed to fractionize the wedged tip of WSF along the propagating direction and introduce rectangular waveguide approximation so that effective index of the segmentations is obtained and SCM analysis model is established. Lightwave propagation and mode field evolution is analyzed by ERA-SCM and compared with finite difference-beam propagation method (FD-BPM), showing that the former is more precise to solve mode field evolution for asymmetric fiber and waveguide structures. The measurement of the output optical spot verifies the ERA-SCM simulation error is within 1.9%, while that for FD-BPM method is 4.5%. Thus, ERA-SCM is an effective method for analyzing asymmetric fiber and waveguide structures.