Stochastic decoding algorithms for non-binary LDPC codes have good performance, but high computational complexity in probability generation. In this paper, we propose a simple method for fast generation of the probability, especially in high-order modulation schemes. The algorithm calculates probability using the quantized Euclidean distances between constellation points and the received signal points. It can avoid the probability calculation for all constellation points and the arithmetic of powers. Simulation results show that the proposed algorithm has low complexity and incurs less performance degradation, which is advantageous in hardware implementation.