Abstract: OmniStar miniature gas mass spectrograph has been used in a vehicle-mounted laboratory and realizes the on-line analysis of H₂, He, CH₄, N₂, O₂, Ar, CO₂, C₃H₈ and C₄H₁₀ with flexible sampling modes according to the actual field condition. Statistical data from 100 consecutive analysis of a mixed standard gas for components of H₂, He, N₂, O₂, Ar, CO₂ at concentration of 100 10^-6 show that the analytical precision is less than 1% RSD and the analytical accuracies (RE) of H₂, He, Ar, CO₂ are 3.74%, 0.94%, 1.83% and 10.5% respectively. Statistical data from 3500 consecutive analysis for the indoor air sample show that the analytical precision (RSD) of H₂, He, CO₂ are 2.25%, 5.55%, 4.06% respectively and analytical precision of N₂, O₂, Ar are less than 1% RSD. Statistical data from 13 consecutive measurements of a mixed standard gas sample for CH₄, C₂H₆, C₃H₈, C₄H₁₀ shows that the analytical precision of C₃H₈ and C₄H₁₀ are less than 1% RSD and the average values for each analyte are in agreement with standard values. High accurate measurement results satisfy the requirements of field onsite analysis. The apparatus along with twist drilling and bag gas sampling has been used in on-line analysis of gases in geological survey regions of Xiwuqi of Inner mongolia and Jinwozi of Xinjiang and the abnormal concentration of CO₂, H₂ and He are found in some sampling sites.