The present invention relates to a process for preparing a porous layer of biocompatible polymer, having a uniform density and porosity, comprising the following steps: a) a quantity Qp of solution of the said polymer, having a viscosity Vp, is poured into a mold in order to form a first sublayer, the surface of the first sublayer being left to the open air; b) a quantity Qs of solvent, having a viscosity Vs, lower than Vp, is spread uniformly over the surface of the first sublayer so as to form a second sublayer; c) the first and second sublayers are subjected to a step of lyophilization, in which the said polymer is a polysaccharide chosen from hyaluronic acid, alginic acid and chitosan, salts thereof and mixtures thereof.