Small reservoir thickness， high crude oil viscosity， large steam heat loss， and short cyclic steam stimulation （CSS） period are problems facing in shallow-thin ultra-heavy oil reservoirs at the early stage of CSS development. Therefore， the development model of large CSS of alternating hot and cold fluid injection was proposed to improve the development effect effectively. In addition， carried out a three-dimensional physical simulation experiment of alternating heat and cold fluid injection in shallow-thin ultra-heavy oil reservoirs. The results show that the temperature of CSS decreases rapidly and the peak production is high because of the heat loss of the top and bottom cap rocks， but the single cycle production time is short， about 100 min. The injection of viscosity reducer can reduce the oil saturation near the well， improve the oil production rate， lower the water content， and increase the CSS period by more than 50 min at the production stage. Increasing temperature can enhance the viscosity reduction effect of the viscosity reducer， and the improvement effect during the second cycle is better than that during the first cycle. In addition， the production time is increased by 60 min， water content is cut by 45%， and the oil recovery is enhanced by 1.7%. The injection parameters of heat and cold fluid were optimized and the policy limits of this technology were established by a numerical simulation method：The best timing of alternating heat and cold fluid injection is the second or third cycle； and the injection strength is 0.02 t/m. The suitable reservoir thickness is less than 8 m； the crude oil viscosity is less than 200 000 mPa·s；the oil saturation is higher than 0.6，and the permeability is higher than 1 000 mD.