The introduction of femtocells brings more interference to cellular system, setting threshold to femtocell users' transmit power can control it effectively. To control the interference from femtocell users to macro base station and guarantee the QOS and maximize system capacity, this paper develops the uplink power control algorithm of femtocell system based on threshold and water- filling theorem. The initial value of transmit power is determined according to the interference suffered by macro base station. The simulation results show that the algorithm maximizes system capacity on the basis of interference control.