In order to mainly solve the problems of reagent dilution and dilution ratio adjustment in a chemical analysis instrument sample injection process, the invention discloses a dilution device able to adjust the ratio and a dilution method. The device comprises: a sample injection pump, an upper liquid level detector, a liquid storage ring, a lower liquid level detector, a two-position three-way mixing valve, a two-position three-way interception valve, a quantification pipe, a two-position three-way sampling valve, a liquid deficiency detector A, a liquid deficiency detector B, a to-be-diluted reagent, distilled water, a waste liquid bucket, a mixing vessel, and a controller. The dilution method includes: controlling the opening and closing states of the two-position three-way interception valve and the two-position three-way sampling valve, intercepting the reagent therebetween in the quantification pipe, then adding distilled water to make the total volume of the intercepted reagent and the added distilled water reach a set volume of the liquid storage ring, and conducting complete mixing, thus completing the dilution process. Specifically, the length and the inner diameter of the quantification pipe and the liquid storage ring can be adjusted to set different dilution multiples. The dilution process is completed in one time, and the volume is fixed after dilution. The dilution process automatically proceeds under the control of the controller. With the dilution device and the dilution method provided in the invention, the structure is simple, the cost is low, the dilution precision is high, and the ratio adjustment is convenient.