The invention discloses a method and device for online monitoring the pulverized coal concentration of coal-bed gas well output liquid.The method includes the steps that a static stratum water density temperature revised value is determined; a flow stratum water density flow revised value is determined; the density of the output liquid under the flow condition is measured; the density of the output liquid under the static condition is calculated and revised; the pulverized coal concentration of a coal-bed gas well stratum is determined.The monitoring device comprises a pulverized coal concentration measuring unit and a measuring information processing unit.The pulverized coal concentration measuring unit comprises a gas-liquid separation tank, an output opening of the gas-liquid separation tank is connected with a density sensor and a flow sensor, a liquid outflow pipe is provided with a temperature sensor, and a liquid outlet of the density sensor is connected with a blow-off pipe.The measuring information processing unit comprises a data acquisition module, a data processing module and a wireless data transmitting module.Labor cost can be reduced, monitoring precision can be improved, online monitoring of pulverized coal concentration of the coal-bed gas well output liquid is achieved, and the method and device are suitable for exploiting a coal-bed gas well.