An intelligent variable-rate orchard sprayer retrofitted with a newly developed premixing in-line injection system was evaluated for its performance in the field. Experimental parameters included combinations of desired chemical concentrations and pulse width modulation duty cycles. To evaluate the dispensing accuracy and discharging accuracy of a metering pump and water pump separately, a flow-rate testing platform was built with two IO-Link-standardized magnetic-inductive flow meters. To assess the chemical concentration accuracy and mixture uniformity, the fluorescent tracer technique was used to analyze the spray mixtures discharged from this system. It was confirmed on the basis of the nominal concentration inputs that chemical concentrates could be accurately dispensed with the metering pump, and water as the chemical carrier could be accurately charged with the water pump. Field-test results showed that the premixing in-line injection system was able to provide acceptable chemical concentration accuracy and desirable mixture uniformity for variable flow-rate spray nozzles. The overall relative error of measured concentrations was 6.36%, and the mean coefficient of variation was 3.09%. The premixing in-line injection system would have great potentials for variable-rate precision sprayers to further reduce chemical waste and environmental pollution.