The electrical potential signals induced by laser-plasma generated on a copper target with bias-voltage using a Nd:YAG laser (7ns) were detected, and the effect of the additional voltage on the potential signals was mainly studied. The experimental results show a single-peak distribution in the temporal profile, which is positive for negative bias voltage and negative peak when with positive bias voltage. Taking into account the effect of plasma charging and the electric field produced by the target bias-voltage, a clear explanation of the observed effect is given.