A polyimide / magnesium hydroxide nano-composite film was prepared by in-situ poly-merization method． A series of magnesium hydroxide nano-composite films with ratio of 0% wt，2% wt，4% wt，6% wt，8% wtwere applied to one research object． We studied the effects of doping amount on dielectric properties of PI / Mg( OH)2hybrid film． In this paper，the dielectric properties mainly include conduction current，electric degradationthreshold and dielectric constant． The results showed that the maximum of conduction current appeared at around4% wt． With the doping content increasing，the electric degradation threshold and dielectric constant all increase gradually．