The present invention relates to polyhydroxyalkanoate synthase (PHA synthase) mutant originated from Pseudomonas sp. 6-19 (KCTC 11027BP) which can prepare lactate polymer and/or copolymer by using lactyl-CoA as a substrate. The present invention relates to a method for preparing lactate polymer and/or copolymer with the synthase mutant. The polyhydroxyalkanoate synthase mutants of the present invention originated from Pseudomonas sp. 6-19 can efficiently prepare lactate polymer and/or copolymer by using as a substrate lactyl-CoA which is difficult to be used as a substrate by conventional polyhydroxyalkanoate synthase.