The invention discloses a gas auto-sample reserving apparatus, an on-line monitoring/synchronous sample reserving system, and an on-line monitoring/synchronous sample reserving method. The gas auto-sample reserving apparatus includes: a pipeline, used for transporting a gas sample; a flow meter, used for measuring the flow rate of a sample in the pipeline; a three-way electromagnetic valve, used for controlling flow direction of the sample in the pipeline and being connected to the flow meter; a needle valve, used for regulating the flow rate of the sample in the pipeline and connected to the three-way electromagnetic valve; a multi-way valve, used for selecting sample reserving pipelines and being connected to the three-way electromagnetic valve; a sample reserving unit, used for reserving the sample and being connected to the multi-way valve. The apparatus achieves on-line analysis of gas and meanwhile performs auto-sampling to the required sample. The reserved sample can reflect authenticity and representativeness of a real-time standard exceeding sample, thereby providing accurate samples for laboratory analysis.