According to the requirements of the tritium related systems of the TBM for monitoring the on-line tritium concentration, a wide-range tritium-concentration detector has been developed to measure the tritium concentration in the range of 104 Bq/ml &sim 5×108 Bq/ml. Correspondingly, the minimum weak current which the detector could be measured is 10−14 A. The performances of the background current and the current response linearity of the prototype have been tested.