The invention discloses a standby roll on a papermaking machine. The standby roll comprises two mounting seats which are symmetrically arranged; a roll shaft is connected between the two mounting seats; one end of the roll shaft is provided with a first motor located in one of the mounting seats; the bottom end of each of the two mounting seats is provided with an air cylinder; the top ends of piston rods of the two air cylinders are connected with a standby roll body; one end of the standby roll body is provided with a second motor; and gaps for vertically regulating the standby roll are formed below the inner sides of the two mounting seats. The standby roll is reasonable in design and suitable for popularization and use.