The free-piston engine generator (FPEG) is a linear energy conversion system, which is known to have greater thermal efficiency than an equivalent and more conventional reciprocating engine. The piston motion of a FPEG is not restricted by a crankshaft-connection rod mechanism, it must be controlled to overcome challenges in the starting process, risk of misfire, and unstable operation. In this paper, the global control structure for a FPEG prototype is presented. A Cascade control strategy is proposed for the piston stable operation level, and PID controllers are used for both of the outer loop and inner loop. The measured top dead centre of the previous stroke and the piston velocity during the current stroke are taken for controller feedback, and the injected fuel mass is used as the control variable. The proposed cascade control implemented in the FPEG is shown to have good performance, the piston returns to a stable state in 0.5s. Compared with a single loop control strategy, the performance of cascade control is improved in terms of the control delay, peak error and settling time