Aiming at the non-electronically controlled single-cylinder diesel mainly applied for agricultural machinery, the electronically controlled CNG gas supply system was only increased to modify the engine but maintaining the original mechanical part. Theoretical analysis and experimental measurement of instantaneous speed for single-cylinder diesel were conducted. The law of single-cylinder instantaneous speed variation was summarized, and based on the law a position determination strategy was proposed. An experimental scheme for CNG-diesel dual-fuel single-cylinder diesel was designed. After implemented the strategy and CNG injection driver on microcontroller, bench tests were carried out. The results showed that position determination based on this strategy achieved good accuracy and the engine ran stably, both under stable and variable engine conditions.