To solve the problem of information leakage of pulse width and frequency possibly caused by pulse intensity modulation in current active decoy-state quantum digital signature scheme, a passive decoy-state quantum digital signature scheme based on linear optical components is proposed. In this scheme, decoy states are passively constructed to estimate channel parameters by making two weak coherent sources to pass a beam splitter and trigger the local detector, so that the research of quantum digital signature can be carried out. This passive decoy-state quantum digital signature scheme can not only avoid the side channel vulnerabilities caused by active pulse intensity modulation and improve the security of the system, but also weaken the effect of modulation error and intensity fluctuation generated in the preparation process of the active decoy-state scheme.