The invention belongs to the technical field of oil drilling, and particularly relates to a shaft overflow early-stage monitoring device and method used for monitoring whether formation fluid (oil and gas) enters the shaft annulus and preventing blowout accidents. The shaft overflow early-stage monitoring device comprises an underground monitoring device and an overground monitoring device, wherein the underground monitoring device carries out real-time measurement and analog-digital conversion on capacitance data, temperature data and pressure data on shaft annulus fluid, then the data are sent to the overground monitoring device, the monitored data are analyzed and processed through the overground monitoring device, and early warning and alarming are carried out. The shaft overflow early-stage monitoring device and method have the advantages of monitoring whether formation fluid enters the shaft annulus in real time through an underground measurement pipe nipple capacitance measurement module, and can accurately judge whether formation fluid enters the shaft annulus at the overflow stage, give an early warning, and provide data support for preparation of well killing materials through the worked out gas invasion amount.