The equivalent transformation between shunt compound stubs and parallel-coupled line, which is the kernel process in the design of bandstop filter with broad upper passband, was discussed in this article, and the process to solve the even and odd impedances of parallel-coupled line from $y$ parameters was presented. To validate the equivalent transformation and the solution process, one design example in which stopband’ fractional bandwidth is 37% at 1 GHz and the upper passband up to 6 GHz was implemented.