An　experimental　study　was　conducted　on　a　spark　ignition　engine　to　investigate　the　factors　that　affect　the　lean　combustion　limit　of　hydrogen-natural　gas　engine.　The　effects　of　ignition　timing,　hydrogen　mixing　ratio,　engine　speed,　throttle　opening,　lubricant　oil　and　coolant　temperature　were　examined.　The　engine　speed　was　1500-3000　r/min,　the　throttle　opening　was　10%-40%,　the　lubricant　temperature　was　75°C-95°C,　and　the　coolant　temperature　was　65°C-85°C　during　experience.　The　hydrogen　mixing　ratio　varied　from　0　to　40%,　and　the　lean-combustion　limit　ignition　timing　was　defined　and　adopted.　The　result　shows　that　both　over-retarded　and　over-advanced　ignition　timing　would　baffle　the　extension　of　lean-combustion　limit.　The　lean-combustion　limit　increases　with　hydrogen　mixing　ratio,　slightly　increases　with　throttle　opening,　and　decreases　when　the　engine　speed　is　raised.　Besides,　it　decreases　with　the　increase　in　lubricant　temperature.　Warming-up　coolant　is　positively　related　to　enlarge　the　lean-combustion　limit.