Dynamic behavior of resources is a non-negligible feature in grid system, and most research efforts on advance reservation cannot effectively deal with the negative effect resulted from the dynamic feature. In this paper, a new grid system architecture using resource pool is proposed firstly. Theoretical analysis demonstrates that resource pool can well adapt to dynamic behavior of resources. Secondly, Quality of Service (QoS) distance computation method for hybrid variable types is presented. Then, k-set Availability Prediction Admission Control (kAPAC) algorithm is described in detail. Experimental results show that kAPAC can significantly increase success ratio of reservation, resource utilization and stability of grid system.