The Al-Zn-Mg-Cu series alloys has been widely used in the aircraft, automotive, ship-building and nuclear industries for the advantages of excellent combination of low density, high strength to weight ratio, good toughness and high corrosion resistance, etc.. Most of the researchers focused on alloying and heat treatment at aging temperature, however, rare work had paid attentions on the deformation process, and the microstructure evolution and mechanical properties has not been analyzed completely. Grain refinement can not only improve the strength and hardness, but also the plasticity and toughness of the alloy. Thermo-mechanical treatment is an efficient and economical treatment for obtain grain refinement by a combination of the deformation and heat treatment. In the present work, an improved thermo-mechanical processing, double step hot rolling (DHR), including low temperature pre-deformation, intermediate short-term annealing and final hot rolling has been proposed, aiming to investigate the microstructural evolution, strain induced precipitation and grain refinement mechanism of the alloys during the DHR process. A 7055 aluminum plate has also been manufactured by the conventional hot rolling (CHR) route. The corresponding microstructure evolution and mechanical properties were investigated by OM, XRD, TEM, SEM, EBSD and tensile test. The results reveal that the grain refinement is mainly preceded via dislocation rearrangement and low angle grain boundaries migration, which in turn leads to the pinning effects of strain induced precipitates. Low temperature pre-deformation can accelerate the formation and spheroidization of fine precipitates. The pre-deformation makes influence on the morphology and average size of precipitates without changing their area fraction. The precipitates generated by the pre-deformation can exert significant drag force to the migration of the grain boundaries and dislocation movements, which subsequently promotes the formation of dislocation cells. Although some smaller particles have been dissolved into the matrix during intermediate annealing treatment, some particles are still fine and can pin the dislocation boundaries. At the same time, the activated dislocation boundaries rearranged to form polygon sub-grains. Grains are further elongated after the final hot rolling. The low angle grain boundaries (like sub-grain boundaries) into high angle grain boundaries transition will be accelerated if the motion of boundaries is impeded by the particles. And the new small grains formed near the original grain boundaries can finally cause the fine-grained structures. The results indicate that the optimum thermo-mechanical treatment of 7055 aluminum alloy may be solid heat treatment+pre-deformation (300 ℃, 20%)+intermediate annealing (430 ℃, 5 min)+hot deformation (400 ℃, 60%). The elongation of the alloy produced by the proposed process can increase by 25% without strength loss comparing with that of conventional hot rolling. And the present DHR process is supposed to be a good alternative manufacturing process for the aluminum alloys to obtain fine grain structured heat-treatable sheets.