Calculation methods for the critical placement height of gravel column reinforced soft soil foundations are not perfect at present. The traditional calculation methods do not take the improvement of composite foundation's strength and the enhancement of foundation's drainage consolidation ability into account. Based on the consolidation theory, the simplified method of consolidation calculation of gravel column composite foundation is introduced, and it modifies the formula of the critical placement height according to the deformation control. Analyzing the settlement deformation observation data of the soft ground distributed along the Suining-Ziyang expressway in Sichuan province, results show that the thickness of clay in the study area is within 6.5-11 m and the observation value of critical placement height ranges from 4.5 to 6.5 m. The difference between observation values and calculation values with modified formula is ±(0.1-0.4)m. It verifies that the formula deduced in this paper is applicable. Based on these, the factors influencing the critical placement height were discussed and it was found that the thickness of soft soil and the column spacing have more obvious influences on the critical placement height than other factors.