Based on measured data obtained from earth pressures on pile - top and sidewise earth pressures between piles in the compound foundation formed by cement mixing piles in the Taipu River Pumping Station, and in combination with static load tests and settlenent monitoring results, the law of pile - soil stress ratio(n) changing with load and the effects of soil quality and pumping station operation on n were respectively analysed. The measared data showed that: the n is increased with load increase under constrution, but begins to be decreased after the pumping station operation; with the same exerting force acting on the top of foundation mat, the more bad the soil quality, the greater the n; the effect of the pumping station operation on n is greatest, at that time, the n equals 1.0; the n is still in dynamically changing at present. 