The invention provides a device for real-time automatic measurement of runoff mud and sand and a measurement method, wherein a measurement scale is installed inside a support shell body; a sampler is installed on the measurement scale; the top end of the sampler is separated from a sample inlet hole; a sample discharge hole is disposed in the bottom end of the sampler; an overflow hole used to calibrate a volume of the sampler is also disposed in the outer side wall of the sampler; a sample inlet valve is installed on the sample inlet hole; the sample inlet valve controls electric motor driving by the sample inlet valve; a sample discharge valve is installed on the sample discharge hole; the sample discharge valve controls the electric motor driving by the sample discharge valve; a first fluid sensor is installed in the sample inlet hole; a second fluid sensor is installed in the overflow hole; and a PLC controller controls the whole measurement device to complete self-starting, self-dormancy and automatic repeated measurement. The measurement device provided by the invention has the advantages that a runoff volume and a content of the mud and the sand in an outdoor runoff small area or at a drainage basin runoff station can be measured automatically and continuously in real time; and labor force consumption of the manual measurement can be reduced. The measurement device and the measurement method provided by the invention can ensure accuracy of measurement results.