To solve the problem of properly mapping the computational task onto a three-dimensional Network-on-Chip (NoC), an improved algorithm based on Genetic Algorithm (GA) was proposed. GA has the fast random searching ability and Prim algorithm can get the minimal spanning tree of a weighted connected graph. By combining the two algorithms' advantages, the improved algorithm could properly assign computational tasks onto each network node, achieving a high efficiency on solving network power consumption and heat problems. The simulation experiments were carried out to compare the proposed improved GA based on Prim algorithm with GA based 3D NoC mapping algorithm. The simulation results indicate that the average power consumption of the improved GA based on Prim algorithm is lower:from the overall trend, the reduction on power consumption is positive correlated to the increase of the number of processing units, and when there are 101 processing units, the average power consumption is 32% lower than that of the traditional GA.