A simplified seismic design model on steel truss coupling beam is suggested based on compatible distortion and balance conditions, considering second order effect and perfect plasticity. This model regards web members of the coupling beam as constraints, thus just the chord need be studied instead of the whole truss. End moments and end shears of chords and relations between distortions of components of the truss and displacement of the coupling beam are established. So curve of load-displacement, plasticity formation and development and influences of area of web members on ductility are convenient to obtain. Results show that this model is more convenient and accuracy than finite element method. Finally some valuable conclusions about influences of span-to-depth ratio and area of web member on ductility of the coupling beam are acquired to contribute to design.