The curves of free photoelectron and shallow-trapped photoelectron decaying with time in different doping method and doping amount of iodide are measured under the action of short-pulsed laser by using microwave absorption phase sensitive technique, which is used to measure faint signal. According to the curve, the lifetime values and the decay time values of free photoelectron and shallow-trapped photoelectron under different condition are obtained. By comparing these values, the work of iodide in AgBr-T grain and its influence on the photographic property of AgBr-T are analyzed. In addition, the absorbed and dispersive signals with the iodide amount less than, more than, and equal 3% are analyzed in detail, and the influencing factors on every section are discussed.