Abstract: To prepare immobilized microbial membrane, the selected fungus of Bacillus subtilis was cultured, and after mixing with a definite amount of supplementary, was applied and sandwiched between 2 pieces of micro-porous cellulose membranes. Assembly of the microbial biosensor detector system for formaldehyde (HCHO) was achieved by association of the immobilized microbial membrane with the dissolved oxygen electrode through attaching and fixing the microbial membrane onto the Au-cathode of the oxygen electrode. To test for response of the biosensor to HCHO, values of out-put current i0 (base-line current) and it (current-decreased after addition of sample solution containing HCHO) were taken, and it was found that values of Δi(i0-it) kept linear relationship with concentration of HCHO in the range from 0.005 to 0.20 g·L-1. Precision was tested with 0.1 g·L-1 of HCHO, value of RSD (n=10) for the response signals was found to be 1.13%. Recovery was tested by standard addition method, giving average recovery of 107%. It was also found that the dried immobilized microbial membrane kept its efficiency for at least 60d when stored in refrigerator at 4 ℃.