The invention discloses a multifunctional drainage automatic gas sample collector for toxic gas and a gas collecting method, which are designed for realizing gas sample collecting standard operation and improving monitoring technology levels. The automatic gas sample collector mainly comprises a bottle plug rack arranged in a sampling pool, sampling and water injection scales, a transverse slide bushing, a transverse slide rod, a horizontal rotating ring connecting the transverse slide bushing with a rotary rack, a longitudinal slide rod connected with the rotary rack and sheathed with a longitudinal slide bushing, a loop clamping a collecting bottle, a pulse magnetic valve connected with an air inducing pipe, an air control valve, and the like. The automatic gas sample collector is a special device, has standard operation, and can obviously improve the sampling success rate; an operator does not contact water directly, thus avoiding the frozen risk completely; the gas collection amount of the collecting bottle can be controlled through the water injection amount of the sampling pool; the collection operation can be controlled automatically, so that the operator avoids the poisoning risk; the sample is got in clear water of the sampling pool, so that the content of the dissolved oxygen in air can be reduced, and the gas sample is prevented from being contaminated.