Aiming at the shortcomings of the existing online measurement methods of syrup brix, a microwave open coaxial resonator sensor with an improved structure was developed, which can be used in the automatic sugar production process. When the syrup mixture comes into contact with the opening of the resonant cavity, the resonant responses (resonant frequency and quality factor) are significantly changed by the material perturbation. By measuring a standard liquid sample with known permittivity, the electric permittivity could be accurately determined. For syrup of 55–90 °Bx, the resonance frequency increases with the increase in brix, and there is a highly reliable quadratic relationship between them. In addition, a quartic relationship exists between the quality factor and the brix; a quadratic relationship between the real part of permittivity and the brix; and a quartic relationship between the imaginary part of the permittivity and the brix. Hence, both the resonant responses of the resonator and the permittivity obtained can be used to evaluate the brix of syrup. Further, the resonant frequency, which had the best correlation, was selected as the index for syrup brix measurement, and standard syrup samples from 60 to 90 °Bx were measured online. The errors between the predicted and measured values were less than 2%, which indicates that the resonant frequency can be used as a reliable index for syrup brix measurement. The syrup brix sensor has the advantages of high accuracy and stability, and can be applied to online measurement of syrup brix in sucrose production to improve the quality of sucrose products. • A microwave method using an open coaxial resonator is proposed for the measurement of the brix of syrup for the first time. • The structure of the sensor has been improved, which makes it have higher measurement accuracy and stability. • Electromagnetic simulation was carried out to reveal the resonant fields.