Foamy oil phenomenon of heavy oil reservoirs gradually disappears in the latter period of solution gas drive, which causes the decrease of oil development. Based on the formation of secondary foamy oil by gas injection, foamy oil unconventional experiments and pressure depletion tests are conducted to reveal the characteristics of the foamy oil and analyze the applicability of various types of foamy oil reservoir simulation models. Evaluation of improving oil recovery by nature gas huff and puff is conducted and the influence of injection and production parameters is studied. The study shows that foamy oil exists quasi bubble point which decreases with the decrease of time. The foamy oil compression coefficient is in the range of 10～0.012 MPa -1, which is higher than that of the conventional crude oil. Foamy oil model of six components is the most suitable model for the foamy oil reservoir. There is optimal gas injection and soak time for nature gas huff and puff. Increasing the gas injection speed, gas injection pressure and fluid production rate will improve the performance of nature gas huff and puff. Increasing the speed that dissolved and injected gas change into disperse gas is very important during the solution gas drive and gas injection process.