The　electronic-controlled　ignition　system　was　developed　for　automotive　engines　fueled　with　compressed　natural　gas　(CNG).　In　order　to　evaluate　the　spark　energy　of　the　electronic-controlled　system　and　optimize　the　electronic-controlled　ignition　system,　referring　to　SAE　J973-1999　standard,　the　spark　energy　measurement　system　was　developed.　Taking　Zener　string　as　simulation　load,　the　voltage　across　the　sampling　resistance　was　acquired　on　time　by　digital　oscilloscope,　then　the　exact　value　of　spark　energy　was　obtained　by　integral　calculations.　The　results　show　that　the　spark　energy　can　be　evaluated　quantitatively　and　effectively.　For　the　electronic-controlled　automotive　CNG　engine　JL465Q5,　when　the　engine　speed　is　lower　than　2500　r/min,　the　optimized　charge　time　of　the　primary　ignition　coil　is　6　ms;　when　the　engine　speed　is　between　2500　r/min　and　5000　r/min,　the　optimized　charge　time　of　the　primary　ignition　coil　is　3　ms;　when　the　engine　speed　is　higher　than　5000　r/min,　the　optimized　charge　time　of　the　primary　ignition　coil　is　2　ms.