The invention relates to a set top box signal output control circuit and a set top box. The set top box signal output control circuit and the set top box detects that if an S/PDIF port is inserted by access equipment, corresponding electrical level is exported according to the detection, judge if there exists insertion of the access equipment according to the output electrical level, the S/PDIF signal will be send out if the access equipment insertion exists and the output of the S/PDIF signal will be stopped if no access equipment insertion exists, and therefore the output of the S/PDIF signal is turned off when the S/PDIF port is not inserted by access equipment or a user decides to turn off the set top box, and the power dissipation of a top set box is reduced and unnecessary electric energy waste is avoided.