The PaPer presents the design of the two-stage depressed collector system for 35GHz gyrotron, taking into account the effect of secondary electrons. Employed a new 2.5-D computer-aided design codes named CHAELOPS, including the auto-gridding generator, the electromagnetic solver, the accurate particle trajectory tracer and the secondary electron emitter including true secondary and backscattered seondary electron models based on Monte-Carlo method. The codes take into account either the safety and the reliability. The user interface is very friendly and fiexible. The collector electrode potentials were optimized in order to maximize the efliciency of the collector system s and minimize the effect of the secondary electrons. The primary beams come from the beam-wave interaction simulation codes. When the potentials were-20kV and -35kV, the magnetic com press ratio was 5.38, the better collector efliciency was 60.0％ with all the secondary electrons landing on the same electrode. The normalized velocity distribution of one trajectory in phase-space was shown in the paper, indicated the theory of the energy conversion of the electrons in retarding electric field and adiabatic magnetic field. The results comparison between EGUN and CHAELOPS has been made and good agreements have been obtained.