Provided are a novel metering system and metering method of hydraulic reaming coal expanding quantity. The metering system comprises road rails paved in a roadway, a mine vehicle moving along the road rails, a drilling machine and high-pressure pump system arranged in the roadway, a settling barrel and a filter barrel which is arranged in the settling barrel and upwards extends out of the settling barrel, and the drilling machine and high-pressure pump system is in transmission connection with a reaming drill pipe and a reaming drill bit. The method comprises the following steps that two sets of the settling barrel and the filter barrel which have the same specification are machined; water-coal-slurry produced through hydraulic reaming is collected; particle coal is settled, and water is drained; powdered coal is collected; the coal expanding quantity is calculated. By means of the novel metering system and metering method of the hydraulic reaming coal expanding quantity, the metering accuracy of the hydraulic reaming coal expanding quantity can be improved, the hydraulic reaming effect is guaranteed to some extent, and therefore the crossing drilling gas extraction effect is guaranteed; promotion of the mine gas control level is facilitated, the coal mine safety production is guaranteed, and application and promotion of the hydraulic reaming new technology are facilitated.