The invention discloses a device for automatically determining the mass concentration of double-channel particulate matters. A particulate matter separation and sampling module is connected with a gas sample humidity control module; a gas outlet of the gas sample humidity control module is connected with a double-channel sampling detection module; two paths of gas outlets of the double-channel sampling detection module are connected with two-path constant-flow regulation module respectively; a gas outlet of the two-path constant-flow regulation module is connected with a sampling pump through a three-way valve. The device provided by the invention can determine the mass concentrations of PM10 particulate matters and PM2.5 particulate matters in atmospheric environment at the same time, has the advantages of simple structure, low maintenance cost, long running period, rapidness and accuracy in data and actual realization on online automatic continuous monitoring, truly realizes the effect that double-channel continuous online sampling process can be finished with only one beta-ray detection device and one beta-ray source and has the remarkably advantages of simple structure, low maintenance cost, long running period, rapidness and accuracy in data, reduction in instrument cost and later maintenance cost and actual realization on online automatic continuous monitoring.