The invention relates to an automatic quantitative sampling device and a sampling method. An XYZ three-axis motion platform in the device is composed of an X-axis mechanism, a Y-axis mechanism and a Z-axis mechanism; a sliding block of the Z-axis mechanism is provided with a sampling clamping jaw; a precision balance, a sample bottle and a sampling pump body frame are placed in front of the XYZ three-axis motion platform; before quantitative sampling, a plurality of sampling screw pumps are arranged in a sampling pump body frame, a sample bottle is arranged at a sampling station, and a test tube is arranged on a precision balance; and during sampling, the sampling clamping jaw takes one sampling screw pump from the sampling pump body frame to move to the sampling station, the Z-axis mechanism drives the sampling clamping jaw and the sampling screw pump to take a certain amount of samples from the sample bottle and then move to the sample placing position of the precision balance to release the samples into the test tube, the precision balance can weigh the weight of the samples in the test tube, and therefore the automatic quantitative sampling process is completed. According to the invention, a sample to be detected can be automatically and quantitatively taken and loaded into the test tube, and compared with the prior art, the sampling accuracy and the sampling efficiency ofthe detected sample can be improved.