A process for preparing hydrolyzed collagen for tanning and retanning leather by using wet-blue leather waste, characterized in that it comprises the steps of: 1) adding liquid caustic soda at 50 °Bé (Baume scale density) to an open stainless-steel reactor, provided with a stirrer, a stream jacket and a set of valves and accessories for its operation; 2) stirring and raising the temperature to 80°C; 3) adding wet-blue shavings or waste while stirring, avoiding excessive foaming by means of the addition of an anti-foaming agent such as Antimussol Wo6 ®, maintaining the temperature at 80°C; 4) increasing the temperature until reaching a boiling temperature once the addition of wet-blue shavings is finished, maintaining the temperature up to 12 hours, wherein the shavings are incorporated in a wet state; 5) carrying out laboratory tests starting at 9 hours of boiling, and then every hour, until 12 hours, to determine the stability of the emulsion and concentration thereof at 10 % of concentration; 6) depending on the previous analysis, maintaining boiling until achieving the minimum of insoluble matter as desired in the emulsion and stability of the emulsion, or: 7) allowing free cooling while stirring, in the reactor, up to 55-65 °C; when this temperature is reached, starting neutralization of the caustic soda with acetic acid until obtaining a pH of 6.8-7.2; 8) adding a bactericidal agent, preferably one based on 4-chloro-3-methylphenol sodium salt such as Preventol WB ®; 9) adjusting the pH, if necessary, between 6.8 and 7.2; 10) adjusting, if necessary, the moisture percentage to 23-27 %, concentration of hydrolyzed collagen to 73-77 %, preferably 75 %, pH from 6.8 to 7.2 and viscosity from 6,000 to 36,000 cps (Brookfield). 1) adding liquid caustic soda at 50 °Bé (Baume scale density) to an open stainless-steel reactor, provided with a stirrer, a stream jacket and a set of valves and accessories for its operation; 2) stirring and raising the temperature to 80°C; 3) adding wet-blue shavings or waste while stirring, avoiding excessive foaming by means of the addition of an anti-foaming agent such as Antimussol Wo6 ®, maintaining the temperature at 80°C; 4) increasing the temperature until reaching a boiling temperature once the addition of wet-blue shavings is finished, maintaining the temperature up to 12 hours, wherein the shavings are incorporated in a wet state; 5) carrying out laboratory tests starting at 9 hours of boiling, and then every hour, until 12 hours, to determine the stability of the emulsion and concentration thereof at 10 % of concentration; 6) depending on the previous analysis, maintaining boiling until achieving the minimum of insoluble matter as desired in the emulsion and stability of the emulsion, or: 7) allowing free cooling while stirring, in the reactor, up to 55-65 °C; when this temperature is reached, starting neutralization of the caustic soda with acetic acid until obtaining a pH of 6.8-7.2; 8) adding a bactericidal agent, preferably one based on 4-chloro-3-methylphenol sodium salt such as Preventol WB ®; 9) adjusting the pH, if necessary, between 6.8 and 7.2; 10) adjusting, if necessary, the moisture percentage to 23-27 %, concentration of hydrolyzed collagen to 73-77 %, preferably 75 %, pH from 6.8 to 7.2 and viscosity from 6,000 to 36,000 cps (Brookfield).