The invention belongs to the technical field of the production of phthalocyanine blue pigments, and particularly relates to a modified phthalocyanine blue pigment for coloring a polylactic acid system and a preparation method thereof. The modified phthalocyanine blue pigment is prepared from the following raw materials in a mixing proportion by weight: copper phthalocyanine, lactic acid, tetrahydrofuran and pure water, wherein the ratio of the copper phthalocyanine to the lactic acid to the tetrahydrofuran to the pure water is 1 to (0.01 to 0.5) to (0.5 to 10) to (0.5 to 10). The modified phthalocyanine blue pigment provided by the invention can be used for coloring a polylactic acid fiber, and further has the following advantages that 1, after a phthalocyanine blue pigment is subjected to surface modification through the lactic acid or an oligomer thereof, a pigment particle has the favorable compatibility with a polylactic resin system, and has the characteristics of being superfine and being easy to disperse; 2, a coloring process is environment-friendly and pollution-free; the pigment is water-fast, can be almost completely utilized during coloring, and is environment-friendly, and no wastewater is generated; 3, as the pigment is uniformly dispersed in a fiber system in the form of a particle, the colored fiber is more better in light resistance, and is difficult in color fading.