Abstract: Analyzing methods,analyzing condition and testing ability of trace deuterated methanes CD_4 in methane were studied.The result shows that the measuring method of CD_4 concentration in atmosphere diffusion tests by pulse injecting quadrupole mass spectrometer(QMS) method is feasible.Pulse injection QMS measuring technology of trace CD_4 was established.On such condition as rinsing-He gases flow 50 mL/min,splitting ratio 1∶100, electron multiplier testing,slow scanning and SIM modeling,the lowest detectable limit of CD_4 in methane is 5.1×10~(-9).Combined with the isotope(separate) technology of CD_4 and methane,the lowest detectable limit of CD_4 in methane is 5.0×10~(-10),and for CD_4 in air,which is 8.5×10~(-15).It is shown that the lowest detectable limit of CD_4 by gas chromatography-quadrupole mass spectrometer method is 40 times of pulse injection QMS method.