Design and optimisation are two important steps for crystallisation process. In this paper, the combined cooling and anti-solvent crystallisation of acetylsalicylic acid (ASA) in ethanol(solvent) and water(anti-solvent) mixture is studied by means of population balance modelling. An average crystal size of 387 is obtained from an optimised temperature, anti-solvent flow rate and residence time profile of three stages crystallisation process based on the steady state optimisation. An attainable region of average crystal size of different residence time is also developed which can provide help for crystallisation process design. Aiming at improving the benefit of crystallisation process, dynamic optimisation of start-up and shut-down process are also developed. By manipulating the anti-solvent flow rate profile, the start-up time is shortened to 86.62% in Strat up optimisation. Besides, the effect of seed on start-up time is also researched in the start-up optimisation. And in shut down process, additional quantity of crystal which meets the standard quality (average crystal size within 95%) is recycled by optimising the anti-solvent flow rate profile. This study shows that the optimisation of temperature, anti-solvent flow rate and residence time will strongly affect the crystal quality. And optimisation of anti-solvent flow rate, seeds and will also provide additional benefits in start-up and shut down of crystallisation process.