Abstract: With the continuous development of economic and building scale, it is more and more attractive to utilize the underground space. There is deep soft soil in floodplain of the Chuhe river. With the expansion of the scale of pit, reasonable physical and mechanical parameters will save costs for the pit protection in the promise of safety, which wiu be good for economic benefits and can offer support for establishing a complete evaluation system for regional soft soil. Analysis for properties of the floodplain soft soil and for adverse effects for the soft soil to construction is done. A modified formula for the in situ testing is proposed and thus a new method to calculate the capacity of the soft soil in floodplain with the in situ testing is suggested.