The present invention provides a low viscosity poly-α-olefin lubricating oil and a synthesis method thereof. The method comprises: (1) the α-olefin raw material is subjected to dehydration treatment so that the water content in the raw material is ≤10 ppm; (2) a reaction of the dehydration treated α-olefin raw material is carried out in the presence of a complex catalyst and gaseous BF3 to obtain a reaction product, wherein the pressure of the gaseous BF3 is 0.01 to 1 MPa; (3) the reaction product obtained in step (2) is sequentially subjected to flash distillation, gas stripping, centrifugation, and washing treatment to obtain an intermediate product; (4) the intermediate product obtained in step (3) is subjected to distillation under reduced pressure to separate the unreacted α-olefin raw material and α-olefin dimers, and the remaining heavy fractions are subjected to hydrogenation saturation treatment followed by fractionation and cutting-off.