The present invention relates to a slurry process for preparing an ethylene polymer having a melt flow ratio F/P, which is the ratio among the melt index value measured according to ASTM 1238 condition “F” and the melt index value measured according to ASTM 1238 condition “P” of equal to or lower than 27, carried out in two or more stages of polymerization at temperatures in the range from 60 to 120° C., in which at least two of the said two or more polymerization stages are carried out under different amounts of molecular weight regulator, said process being carried out in the presence of (A) a solid catalyst component comprising Ti, Mg, halogen, having a porosity (PF), measured by the mercury method and due to pores with radius equal to, or lower than, 1 μm, of at least 0.3 cm3/g and a surface area determined by BET method, of lower than 100 m2/g, and being further characterized by the fact that more than 50% of the titanium atoms are in a valence state lower than 4 and (B) of an organoaluminum compound.