To understand the thermal evolution of lacustrine sedimentary n‐alkane hydrogen isotopic composition (δD), especially bacterially derived n‐alkanes, anhydrous thermal simulation experiments were performed with sediments from Lake Gahai (Gannan, China). We analyzed the original and pyrolysis‐generated n‐alkanes and their δD values. The results showed that thermal maturity and n‐alkane origins significantly affected the distribution of pyrolysis‐generated n‐alkanes. In immature to post‐mature sediments, the bacterial‐derived medium‐chain n‐alkanes generally had depleted δD values. The maximum difference in average δD values between the bacterial‐and herbaceous plant‐derived medium‐chain n‐alkanes was 32‰, and the maximum difference in δD values among individual n‐alkanes was 59‰. We found that the average δD value of pyrolysis‐generated n‐alkanes from different latitude was significantly different in immature to highly mature sediments, but similar in post‐mature ssediments. The hydrogen isotopes of sedimentary n‐alkanes can be used as indicators for paleoclimate/paleo‐environment conditions only when sediments are immature to highly mature. During thermal evolution, the δD value of generated individual n‐alkanes and the average δD value increased with thermal maturity, indicating that hydrogen isotopes of sedimentary n‐alkanes can be used as an index of organic matter maturity. We established mathematical models of average δD values of generated n‐alkanes from immature to post‐mature sediments using nC₂₁^−‐/nC₂₁⁺ and average chain lengths. These results improve our understanding of the distribution and δD value of sedimentary n‐alkanes derived from herbaceous plants in mid‐latitude plateau cold regions.