A spectrometer engine and an adjustment method thereof are provided. The spectrometer engine includes a connector, a light sensor, a variable gain amplifier, a variable reference voltage generation circuit, an analog-to-digital converter and a control circuit. The light sensor senses a light to be measured coming from an object to be measured to generate a sensing signal. The variable gain amplifier amplifies the sensing signal according to a first setting parameter to generate an amplified signal. The variable reference voltage generation circuit provides a reference voltage according to a second setting parameter. The analog-to-digital converter converts the amplified signal to a digital signal according to the reference voltage. The control circuit reads the digital signal and adjusts at least one of the first to third setting parameters according to the digital signal for the spectrometer engine to measure the object to be measured again to generate another digital signal.