The formation is swept incompletely at plane and vertically at later stage of cyclic steam for thick heavy-oil reservoir. There is little potential in reservoir with cyclic steam due to little exploitable reserve of single well. Based on the steam injection mode, the paper has utilized thermal numerical simulation to complete the development system including injection-production system, seepage mechanism and optimization of parameters including horizontal distance between dual horizontal well and vertical well, vertical distance of dual horizontal well and length of horizontal interval. The results reveal that both steam injection by upper horizontal well and vertical well production are critical for the whole system, while lower horizontal wells accommodate the system by flowing back to achieve ideal production. The vertical-horizontal development has become major issue due to its feasibility for further improvement of sweeping volume by steam injection and that of recovery at thick heavy oil reservoir. The producing reserves of reservoir is enhanced due to enhancing the plane sweeping efficiency of the steam with the vertical well and the vertical sweeping efficiency of the steam with the low horizontal well. Optimal development for thick heavy oil reservoir could be obtained with horizontal distance between dual hori? zontal well and vertical well of 50 m, vertical distance of dual horizontal well of 15 m and horizontal section length of 350-400 m.