The invention relates to a pulse constant-current source and direct constant-current source superposed circuit which comprises a heavy current pulse constant-current source forming circuit and a low current pulse constant-current source forming circuit. The heavy current pulse constant-current source forming circuit comprises a heavy current voltage source, a pulse constant-current source controlcircuit and a pulse constant-current source control switch, and the low current pulse constant-current source forming circuit comprises a low current voltage source, a direct constant-current source control circuit and a direct constant-current source control switch. Negative electrodes of both voltage sources are grounded respectively and independently, a superposed switch is connected between two grounded terminals, and a control terminal of the superposed switch is connected with a control circuit controlled by the superposed switch. An adverse current preventing element is connected between the superposed switch and the negative grounded terminal of the heavy current voltage source and/or between the superposed switch and the negative grounded terminal of the low current voltage source, thus effectively avoiding interference on low direct current due to rising edge and falling edge of the heavy current pulse, and avoiding damage to the low current voltage source for the heavy pulsecurrent is provided with the low current voltage source.