Modal propagation analysis (MPA) and exact mode analysis (EMA) are compared in this paper.Analysis on basic principles and applications of these two methods are performed.As an appoximate method,MPA is convenient and simple,however,it has more errors on propagation constants and field distributions of high order modes.Thus it is not accurate enough when used on devices which acquire low output at one or more output ports.And EMA,which is an accurate method despite of its complicated process,should be used at this time.