The present invention provides a circuit for obtaining time for photons to reach a detector and a detector. The circuit comprises a first comparator, a second comparator and a time determining chip; the first input end of the first comparator and the first input end of the second comparator are both connected with input signals; the second input end of the first comparator is connected with a first voltage threshold value; the second input end of the second comparator is connected with a second voltage threshold value; the first voltage threshold value is larger than the second voltage threshold value; the output end of the first comparator is connected with the first pin of the time determining chip; the output end of the second comparator is connected with the second pin of the time determining chip; the clock end of the time determining chip is connected with synchronization time signals; and the time determining chip is used for performing logical operation on the output signals of the first comparator and the output signals of the second comparator under the synchronization time signals so as to obtain time information. The circuit can accurately obtain the time for the photons to reach the detector, is low in cost and small in size.