An approach for safe diagnosis of timed discrete-event systems(TDESs) is proposed. Firstly, the notion of safe diagnosability of TDESs is formalized. Then by constructing the recognizer of illegal language, the sequences of the forbidden unsafe operations are identified, and the safe diagnoser is constructed to perform the safe diagnosis. In particular, a necessary and sufficient condition for safe diagnosability is deduced, and the safe diagnosis of TDES is realized.