Abstract: Complex structure well，which increases the steam effective displacement area，can greatly enhance oil recovery by developing heavy oil reservoir.The study on flow mechanism of complex structure well is far behind the field application.It is difficult to establish the productivity prediction model and obtain analytical solution for complex structure well considering many factors.Forecasting results differ significantly from the actual production results especially for complex structure well in thermal recovery.By using gray relational approach to the initial production as a reference sequence，reservoir thickness，permeability，branch well length，well spacing，angle，injection parameters etc.are regarded as comparison sequence according to the actual data，the correlation degree and controlling factors are obtained in this paper.The main controlling factors are optimized for complex structure well of heavy oil using steam huff and puff.The research results will provide the necessary technical reference for complex structure well in thermal recovery.

Key words: complex structure well, steam huff and puff, productivity, influencing factor, grey relation, optimization