Molecular characterization of low-density polyethylene has long been faced with many challenges. Due to the lack of control over the radical polymerization under very high pressures, the low-density polyethylene (LDPE) molecular structure is very complex. This paper uses the attenuated total reflectance (ATR)-Fourier transform infrared characterization method to introduce a measuring tool to calculate the average number of branches in LDPE molecular chain. Among 28 gel permeation chromatography (GPC)-analyzed LDPEs, two GPC-identical tubular LDPE were chosen. The branching structures of the LDPEs are investigated by the van-Gurp Palmen plot. To clarify the difference between rheological behavior of LDPEs, GPC, temperature rising elution fractionation, ATR, dynamic oscillatory shear, and elongational stress growth coefficient tests were performed. It was found that high molecular weight portion of LDPE 2 has higher number of branches than LDPE 1, which is responsible for the different rheological behavior.