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Power allocation has a significant impact on sum rate in non-orthogonal multiple access (NOMA) systems. In this paper, we propose a dynamic power allocation (D-NOMA) strategy that can optimize the sum rate compared with the fixed power allocation (F-NOMA) method in uplink NOMA systems. In particular, we first design an evaluation model to analyze several key constraints for optimization objectives, and then derive the optimal power allocation coefficients. The simulation results demonstrate that the proposed strategy outperforms the existing fixed power allocation strategies with different coefficients and occupies more advantage on the low SNR than other D-NOMA scheme in typical uplink pairing scenarios.