The invention discloses a polyacrylic acid synthesis reaction apparatus and a polyacrylic acid synthesis method which belong to the technical field of chemical equipment and synthesis. The apparatus consists of a micro mixer, a reaction pipeline with a heat exchange jacket, and a back pressure valve. According to the method, acrylic acid solution and initiator solution are respectively introduced into the micro mixer from corresponding inlets and are mixed according to a set proportion in the micro mixer, and then polymerization is carried out in the reaction pipeline to obtain a polyacrylic acid solution, wherein the reaction temperature is controlled by the heat exchange jacket and the pressure is controlled by the back pressure valve. According to the invention, high stability of the product is obtained by enhancement mixing and continuous synthesis, the molecular weight is narrow in distribution and is easy to regulate and control in size, and the polyacrylic acid synthesis reaction apparatus and the polyacrylic acid synthesis method have a wide popularization prospect.