A fuel cell system comprising: a fuel cell stack ; a first cooling line having first cooling water that passes via the fuel cell stack and circulates therein; a first radiator disposed on the first cooling line and configured to cool the first cooling water; an air conditioning system disposed on a first connection line that forms a heating loop with the first cooling line ; a first cooling fan configured to blow exterior air to the first radiator ; a first valve configured to switch a flow path of the first cooling water to the fuel cell stack or a heater ; a second valve configured to switch the flow path of the first cooling water to the fuel cell stack or the first radiator ; a first pump disposed on the first cooling line and configured to pump the first cooling water; and a controller connected to the first cooling fan , the first valve , the second valve , and the first pump , wherein the controller is configured: to detect a failure of the first cooling fan ; and when a failure of the first cooling fan is detected, to open the first valve such that the first cooling water flows toward the fuel cell stack ; to control an RPM of a blower of the air conditioning system to a maximum level; to control an opening degree of the second valve according to a cooling degree of the first radiator and a cooling degree of the air conditioning system ; and to control an RPM of the first pump to a maximum level. a fuel cell stack ; a first cooling line having first cooling water that passes via the fuel cell stack and circulates therein; a first radiator disposed on the first cooling line and configured to cool the first cooling water; an air conditioning system disposed on a first connection line that forms a heating loop with the first cooling line ; a first cooling fan configured to blow exterior air to the first radiator ; a first valve configured to switch a flow path of the first cooling water to the fuel cell stack or a heater ; a second valve configured to switch the flow path of the first cooling water to the fuel cell stack or the first radiator ; a first pump disposed on the first cooling line and configured to pump the first cooling water; and a controller connected to the first cooling fan , the first valve , the second valve , and the first pump , wherein the controller is configured: to detect a failure of the first cooling fan ; and when a failure of the first cooling fan is detected, to open the first valve such that the first cooling water flows toward the fuel cell stack ; to control an RPM of a blower of the air conditioning system to a maximum level; to control an opening degree of the second valve according to a cooling degree of the first radiator and a cooling degree of the air conditioning system ; and to control an RPM of the first pump to a maximum level. to detect a failure of the first cooling fan ; and when a failure of the first cooling fan is detected, to open the first valve such that the first cooling water flows toward the fuel cell stack ; to control an RPM of a blower of the air conditioning system to a maximum level; to control an opening degree of the second valve according to a cooling degree of the first radiator and a cooling degree of the air conditioning system ; and to control an RPM of the first pump to a maximum level. <EOS>