In this study, a simulated annealing algorithm is proposed as an optimisation method for the mirror arrangement of heliostat mirrors in a tower solar thermal power plant. Firstly, the optimal number and position of heliostats in the whole mirror field in radial staggered layout (EB unobstructed layout) were solved using simulated annealing method. Then further optimisation, i.e. changing the mirror width, mirror height and mounting height of the heliostats and solving for the optimal number and position of heliostats using simulated annealing again, resulted in an increase in the number of heliostats in the optimised mirror field by 77.92% but a decrease in the total area by 18.77%, and an increase in the average annual thermal power output per unit area of the mirrors by 4.76%. Therefore, this result shows that the heliostat field optimised using the simulated annealing algorithm has a higher efficiency and a smaller footprint.