Effect of steam flooding is not good for easy steam channeling and low steam sweep efficiency. In order to seek out a new way to enhance recovery factor in heavy oil reservoirs，a field test of foam-assisted steam flooding was made in Zhongerbei Ng5 Block of Gudao，Shengli Oilfield. According to the high temperature of steam flooding，a new type of foaming agent DHF-1 was developed. The agent has good performance of temperature resistance and plugging，and resistance factor under 300 ℃ is more than 20. Laboratory experiment result of oil displacement shows that foam-assisted steam flooding can improve sweep volume and oil displacement efficiency. Field experiment of the foam-assisted steam flooding was made in October，2010. The injecting pressure of the steam injection well rose by 1.6 MPa averagely. All oil wells achieved good results. The application result of tracer show that the flooding sweeping is more homogenous，and the average oil displacement efficiency rose to 62.4% based on the data of sealed coring wells. It shows that the foam-assisted steam flooding can improve the stability of steam front，control steam breakthrough and improve development effect obviously. By the end of June，2016，the cumulative oil production has been 23.6×104 t and the recovery degree is 52.1% in the pilot area，and the recovery efficiency has been improved by 16.8% compared with steam huff and puff.