A latch structure of fire door lock includes a lock base, a latch unit, a blocking member, an elastic member and a linking set. The lock base comprises an accommodating slot, a first lateral portion having a first sliding slot, and a second lateral portion having a second sliding slot. The latch unit has a combining pin. The blocking member is disposed in the accommodating slot and includes a contacting portion. The contacting portion is pressed by the elastic member. The combining pin is connected at the linking set and the latch unit. Two end portions of the combining pin are slidably disposed at the first sliding slot and the second sliding slot respectively. The end portions of the combining pin are restrained within a first recess of the first sliding slot and a second recess of the second sliding slot respectively.