Abstract: On the basis of qualitative analysis of the transient ionizing radiation function errors for SRAM based FPGA, the function error threshold was considered to be independent of the quantity of sensitive configuration bits. Different shift registers made up of look up tables and D flip-flops with quite different quantity of sensitive configuration bits were designed. Experimental investigations were performed using pulse gamma ray generated by “Qiang GuangⅠ” accelerator. Comparing with the function status of those circuits under different dose rates, experimental results show that no evidence of the number of sensitive configuration bits affects the function error threshold, and the qualitative analysis is verified.