Abstract: The power recovery ability of a turbocharged two stroke spark ignition(SI) engine at altitude was studied experimentally at ground and simulated at altitude.A new exhaust system was applied for the purposes of achieving good scavenging performance.Through proper design of the exhaust pipe's length and cross section area variation,scavenging performance was improved,but always deteriorated on turbocharged engine because of back pressure rising in the exhaust pipes.Turbocharged two stroke spark ignition(SI) engine is better than corresponding original engine at altitude both in terms of output and economics.So,it is suitable to work at high altitude.