Abstract: It is very important to accurately measure tritium concentration and morphology for mastering tritium release behavior of tritium production breeder and improving the performance of tritium production breeder in out-of-pile tritium release experiment. According to the characteristics of small flow of carrier gas, small quantity of gas and argon as carrier gas, based on the ionization chamber, the digital online tritium measurement system was developed. The sensitive volume of ionization chamber is 50 mL, the digital instrument can automatically obtain the tritium concentration, at the same time, process and display it. The results show that the saturated zone of ionization chamber is about 35 V in argon, and the detection limit is 3.7×10 7 Bq/m 3, which is satisfactory for online tritium measurement in out-of-pile tritium release experiment.