The present invention discloses a low constant bath ratio cotton yarn dyeing method for bobbin yarns and warp beam cotton yarns. The dyeing process comprises stages of laboratory sampling, pre-treatment, dyeing and post-treatment. It is ensured that the actual bath ratio value during the dyeing stage is equal to the operation bath ratio value during the laboratory sampling stage, the operation bath ratio value during the laboratory sampling stage is 1:5, the operation bath ratio value during the pre-treatment stage is 1:4.34, the operation bath ratio value during the dyeing stage is 1:2.84, all the operation bath ratio values are low bath ratio values. With adopting the dyeing method, the dyeing color yield difference regularity between the laboratory sample yarn and the dyeing workshop bulk yarn can be effectively summarized.