In order to study the gas accumulation mechanism of lower Paleozoic in the north of Tianhuan depression, using exploratory well data and organic geochemical methods, the characteristics of Ordovician source rocks were evaluated and their maturity evolution history was restored. Organic carbon content and hydrocarbon potential of source rocks from the Wulalike and the Kelimoli Formations are relatively high, the source rocks are mainly mediumgood and a few are poor. The source rocks of the Sandaokan and the Zhuozishan Formations are mainly poor. The organic matter types of source rocks are mainly kerogen type Ⅱ1~I, with high degree of thermal evolution, in the dry gas stage. Simulation of mature evolutionary history shows that the Palaeozoicearly Mesozoic geothermal gradient was low, the burial depths of the Formations were shallow, and the degree of maturity and evolution of source rocks was low. In the Yanshan period, due to tectonic thermal events, the geothermal gradient increased rapidly, and maximum temperature experienced by the early Cretaceous source rocks exceeded 200 ℃, reaching the peak of gas generation. Early Cretaceous is the main period of hydrocarbon accumulation. After Early Cretaceous, Ordos basin uplifted, geothermal gradient decreased and hydrocarbon generation stopped.