The interspinous spacer (ISP) is a minimally invasive surgical device implanted into the interspinal space to treat lumbar degenerative diseases. Unfortunately, ISPs sometimes cause device breakage and spinal process fracture. Our aim was to elaborate the design of lumbar customized posterior fixation system (CISP system), encompassing a customized ISP body and transfacetopedicular screws, and examine its biomechanical effect on the lumbar spine using finite element (FE) analysis. We constructed the CISP system, based on the interspinal anatomical data at the surgical level. We generated the L3-S1 FE models, implanted with the polyetheretherketone (PEEK) CISP system, titanium alloy (TI) CISP system, and Coflex device at the L4/L5 segment, and determined the lumbar segmental range of motions (ROMs), intervertebral discs (IVD) peak stress, and implant stresses. The CISP system enhanced mobility restriction at the surgical level, compared to the Coflex device. Furthermore, the IVD peak stress reduction was more obvious in the CISP system than the Coflex device, particularly during extension. Under the same motion mode, the maximum stress on the TI CISP system was smaller than on the Coflex device, but larger than the PEEK CISP system. Given these evidences, PEEK appeared to be a better material for the CISP body.