Foam fluid has good property, widely used in petroleum engineering. Density of foam fluid is very important in site application. Device designing used to measure density of foam fluid was introduced under underhole situation. Method to quantitatively determine the density-pressure-temperature relation of the foam fluid was also put forward. The relations between the density of the foam fluid and the pressure and temperature were measured using the method. Experiential formulas calculating density of foam fluid were gained through regression method. The results of measurement and conventional calculation were compared. It shows that density of foam fluid increases with pressure increases, and decreases with temperature increases. The larger the ratio of gas and liquid weight in foam fluid is, the better the condensability of foam fluid is. The measured density of foam fluid is always less than that of calculated by conventional method. Difference lies in 1.45 %to 11.31%. The reason is that repellent force of surfactant ions has not been considered when calculating by conventional method. The regressed formulas have good accordance with the experimental data. The research conclusions can direct foam fluid application in oil field.