A method for manufacturing an intermediate prepreg, intended to form at least one portion of a finished part, said prepreg being made from a composite material comprising a matrix made from a thermoplastic material and reinforcing fibers embedded in the matrix, the method comprising the following consecutive steps: - providing at least one reinforcement layer and at least one matrix sheet made of thermoplastic material; - forming a complex with said reinforcement layer and said matrix sheet; and - placing the complex in a prepreg production tool, the production tool being heated to a temperature higher than the melting temperature of the thermoplastic material so that the thermoplastic material impregnates the fibers of the reinforcement layer(s) in order to form a prepreg; the method comprising, in the production tool, the following consecutive steps: - heating the complex to a temperature higher than the melting temperature of the thermoplastic material without applying pressure on said complex, - heating the complex to a temperature higher than the melting temperature of the thermoplastic material while applying a production pressure higher than atmospheric pressure on the complex, the method for manufacturing being characterized in that it comprises the following consecutive steps, before placing the complex in the production tool: - placing at least the matrix sheet in a treatment tool, - heating said sheet to a temperature lower than the melting temperature of the thermoplastic material. - providing at least one reinforcement layer and at least one matrix sheet made of thermoplastic material; - forming a complex with said reinforcement layer and said matrix sheet; and - placing the complex in a prepreg production tool, the production tool being heated to a temperature higher than the melting temperature of the thermoplastic material so that the thermoplastic material impregnates the fibers of the reinforcement layer(s) in order to form a prepreg; the method comprising, in the production tool, the following consecutive steps: - heating the complex to a temperature higher than the melting temperature of the thermoplastic material without applying pressure on said complex, - heating the complex to a temperature higher than the melting temperature of the thermoplastic material while applying a production pressure higher than atmospheric pressure on the complex, - heating the complex to a temperature higher than the melting temperature of the thermoplastic material without applying pressure on said complex, - heating the complex to a temperature higher than the melting temperature of the thermoplastic material while applying a production pressure higher than atmospheric pressure on the complex, the method for manufacturing being characterized in that it comprises the following consecutive steps, before placing the complex in the production tool: - placing at least the matrix sheet in a treatment tool, - heating said sheet to a temperature lower than the melting temperature of the thermoplastic material. - placing at least the matrix sheet in a treatment tool, - heating said sheet to a temperature lower than the melting temperature of the thermoplastic material.