To address the issues of coarse-grained thread allocation and difficult load balancing in compiler automatic parallelization for processors, this paper proposes a loop-block-level automatic parallelization method for compilers based on an iterative compilation mode, using the SWGCC compiler on the Sunway platform. An automatic parallelization method that independently sets the number of threads for each loop block is designed within the SWGCC, which allocates threads to each parallelizable loop block in the program at a finer granularity. Meanwhile, iterative compilation is combined with a genetic algorithm to iteratively optimize the optimal thread group. Through operations such as the chromosome encoding of thread allocation schemes, weighted mutation operations based on loop execution proportions, and fitness function-guided population evolution, the optimal thread combination is efficiently searched for the loop block thread allocation algorithm. Experiments are validated on the Sunway processor using the SPEC2006 test suite. The results reveal that the loop-block-level compiler automatic parallelization algorithm combined with the evolutionary algorithm achieves a maximum performance score improvement of 19% and an average performance score improvement of 4% compared to the baseline automatic parallelization algorithm in the tests.