The invention discloses an electrospray ion source, which comprises a hollow capillary tube, a vacuum chamber, electrodes and a direct-current power supply. One end of the hollow capillary tube is used as a sampling end, the other end of the hollow capillary tube is used as an electrospray nozzle, the nozzle and the sampling end are arranged in different pressure environments, the nozzle extends into the vacuum chamber, the pressure for the nozzle is lower than the pressure for the sampling end, the electrodes are arranged in the vacuum chamber and behind the nozzle and are provided with a channel for electrospray to pass through, and the direct-current power supply is applied between the hollow capillary tube and the electrodes. The electrospray ion source can be used as a simple ionization device to generate stable ion flow, and is particularly suitable for use as an ion source of a portable mass spectrometer, and the true real-time online field detection and analysis and electrospray ionization can be realized.