The invention discloses a constant temperature and constant pressure reactor. A jacket or a tube group is arranged on the side wall of the cylindrical tank of the reactor, a liquid is poured into thejacket or the tube group, a liquid inlet and a liquid outlet are formed at the lower end of the reactor, a steam outlet is formed at the upper end of the reactor, a liquid outlet valve is arranged onthe liquid outlet, a liquid inlet adjusting valve is arranged on the liquid inlet, and the liquid inlet adjusting valve is connected with the liquid level meter of the jacket or the tube group througha control system, thereby realizing the automatic liquid feed; an air exhaust adjusting valve is arranged on the steam outlet, and the air exhaust adjusting valve is connected with the pressure gaugeof the jacket or the tube group through a control system, thereby realizing the automatic air exhaust. The air exhaust adjusting valve communicates with a steam package through an air tube, and the steam package communicates with a rectification tower through an air tube. With the advantages of reasonable design, easy operation, good effects of constant temperature and constant pressure, high efficiency and high energy utilization rate, the constant temperature and constant pressure reactor is widely applicable to the reactors which radiate heat during reactions in the chemical field, in particular to the reactors used for producing furfuryl alcohol, and has great economic and social benefits.