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The coalbed methane (CBM) resources are abundant in low-rank coal (R o < 0.70%), which has raised widespread concerns within domestic geologists. In brief, low-rank CBM resources are still at an early stage of exploration in China, and the study on adsorption mechanisms and control factors of low-rank CBM is not systematic. As an important geological event of dividing brown coal and bituminous coal, “first coalification jump” has an obvious control action to the coal composition and pore structure of coal reservoirs and the occurrence and migration modes of methane gas, and the targeted research remains lacking. In this paper, the control function of coal rank to reservoir properties was analyzed first and the role of “first coalification jump” was emphasized in the discussion. In addition, in combination with methane adsorption isotherm experiments under equilibrium moisture (30 °C), internal factors (e.g., coal rank, specific surface, macerals, volatiles, and moisture) that control methane adsorbability of the coal were discussed systematically. Studies have shown that methane adsorbability of low-rank coal was mainly affected by adsorption space along with moisture sharp decreases but not Brunauer–Emmett–Teller (BET) specific surface increases under the dominant control function of coal rank.