Structural principle of single cylinder axial mechanical-hydraulic confined piston engine (MHCPE) was described. Its working process mathematical model considering fluid, solid and gas coupling was created, and initial solving condition was given also. By the definition of “external characteristic of MHCPE”, the external character simulation was carried out aiming at single cylinder axial MHCPE developed. The simulation results showed that, output pressure is obviously directly proportional to hydraulic energy ratio, but slightly be influenced by rotational speed; output flow increases firstly, decreases then as rotational speed increases, but slightly be influenced by hydraulic energy ratio; output torque is obviously inversely proportional to hydraulic energy ratio, but slightly be influenced by rotational speed; fuel consumption is very bad when rotational speed and hydraulic energy ratio is great; total effective work turns less slightly versus hydraulic energy ratio, and the trend is very obvious at high rotational speed.