Nanoscale encapsulated carbon black was prepared by phase separation method using branched styrene-maleic anhydride copolymer. The effects of dropping speed of phase separation agent, amounts of BPSMA, pH, mass ratio of carbon black and dispersing time on particle size of carbon black were investigated. FTIR、TEM and Cantact angle analysis confirmed that BPSMA was successfully encapsulated on the surface of carbon black. The optimal preparation conditions of nanoscale encapsulated carbon black were as follows: the dropping speed of phase separation was 3mL/min, amount of BPSMA was 20%, pH was 10, mass ratio of carbon black was 8% and dispersing time was 10min. It could be found that the particle size was smaller and stability was higher, the K/S value and rubbing fastness of the printed fabric with dispersion which was prepared by BPSMA was higher than that of by linear styrene-maleic anhydride polymer.