The modular high temperature gas-cooled reactor (MHTGR) based nuclear steam supplying system (NSSS) is constituted by an MHTGR, a once-through steam generator (OTSG) and produces superheated steam flow for electricity generation or process heat. Although the current PID control law can guarantee satisfactory closed-loop stability, which regulates the neutron flux, primary coolant temperature and live steam temperature by adjusting the control rod speed as well as primary and secondary flowrates. However, the thermal power of NSSS needs to be further optimized. Motivated by this, a dynamic matrix control (DMC) is presented to optimize thermal power of the MHTGR based NSSS. A step-response model with the thermal power response data is utilized in designing the DMC. The design objective of DMC is to optimize the deviation of the thermal power from its reference under its rate constraint. Then, the DMC is applied to the thermal power control, whose implementation is given by forming a cascade control loop with the PID in the inner loop for stabilization and with DMC in the outer loop for optimization. Numerical simulation results show the satisfactory improvement of thermal power response. This cascade control structure inherits the advantages of both PID and DMC, by which the zeros offset and the short settling time of thermal power are realized.