Abstract: Combined measurement error of average photosynthesis rate is comprised of the instrumental error．the measurement error and the characteristics of leaf photosynthesis．The instrumental error comes from CO₂ analyzer，flowmeter．temperature thermometer and barometer．The error is different with different measuring methods or leaf photosynthesis rates．The photosynthesis rate is stronger，the error is smaller．The error in open system is about 8％～ 20％，and is about 2．5％～ 15％ in closed system ．The main eror is owing tO CO₂ analyzer and flowmeter．