A reciprocating piston expander model based on organic Rankine cycle (ORC) is built for engine waste heat recovery. The expander characterizes by variable expansion ratio through adjusting refrigerant injection timing. This paper investigates the effect of refrigerant evaporating pressure, expansion ratio and clearance volume on the expander performance which mainly includes output power, expander efficiency, equivalent recovery efficiency, total output power, expander efficiency, weighted efficiency of expender and weighted heat recovery efficiency of expender. The results demonstrate that the total output power and the equivalent heat recovery efficiency increase with refrigerant evaporating pressure under overall operating conditions, while the increment is negligible. The expander reaches maximum total output power up to 83.442kW under c100 engine condition and 1.1MPa refrigerant evaporating pressure within the research operating conditions. The peak values of total output power and equivalent recovery efficiency are 19.894kW and 31.221% respectively when the expansion ratio reach six. The above two indicators reduce with further increasing or decreasing expansion ratio for over-expansion or under-expansion respectively. The expander performance deteriorates as clearance volume increase due to the reduction of effective expansion ratio. Generally, the variable expansion ratio expander has an excellent performance in waste heat recovery from automobile.