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Today’s common current-output temperature and humidity transmitters have some deficiencies such as poor characteristic of constant current output,low accuracy and cumbersome calibration. This paper presents a design of the transmitter based on STM32 microcontroller in order to make up for the deficiencies. With the STM32 as the core controller,the circuit gains the temperature and humidity signals by SHT11 digital sensor. The signals are converted to analog voltage signals via the internal 12-bit D/A module of STM32 microcontroller. After which,the voltage signals become 4 ~ 20 mA analog current signals through the V/I converter circuit. This paper studies the principle of the V/I converter circuit and then optimizes the circuit. At last,this paper presents a calibration scheme based on PC software. The simulation and experimental results show that the transmitter has advantages such as good constant current characteristic,high precision,easy and fast calibration. These superiorities of the transmitter meet the requests of conventional temperature and humidity measurement in industrial and agricultural production.