The invention discloses a pre-treatment process for dying cotton and a cotton elastic fabric. The process comprises a scutching step, a cloth boiling step and a biological enzyme bio-washing step, wherein in the biological enzyme bio-washing step, 0.5 to 2 g/l of cellulase and NaAc-HAc buffer solution are used; the NaAc-HAc buffer solution consists of 0.1 to 1 g/l of HAc and 0.5 to 2 g/l of NaAc; the treatment bath ratio of the cotton or the cotton elastic fabric to enzyme is 1:3.5-5.4; temperature is between 50 and 85 DEG C; and time is between 30 and 50 minutes. The NaAc-HAc buffer solution serving as an aid is added in a biological enzyme bio-washing process, a bio-washing environment is controlled through the aid and the pH value of the bio-washing process is controlled to be between 4.0 and 4.5 to form a bio-washing condition most suitable for the activity of the cellulase, so that bio-washing effect can be improved, cloth cover quality is higher, one-time dying success rate of a product can reach over 98 percent, even 99.5 percent, and pilling test GB-T4802.3 is up to more than grade 3.5, even grade 4. Moreover, bath ratio is lowered to be 1: 3.5-5.4, water is saved and cost is lowered.