This study is aimed to use shear wave elastography (SWE) to study the relationship between shear modulus and different body postures of the thoracolumbar fascia (TLF) and acquire physiologically meaningful information from the stiffness-posture graph to better quantify passive flexion responses. Seven passive postures were defined to evaluate the shear modulus of right side TLF at the third and fourth lumbar vertebra levels (L3 and L4) in twenty healthy male subjects. The TLF stiffness was significantly different among different postures (p < 0.001), and the TLF stiffness at L3 was always less than that at L4 (p < 0.001). As the forward tilt increased from 0 to 60°, the TLF stiffness increased in sitting and standing postures by 54.01% and 192.84%. In the neutral postures, the TLF stiffness in standing and sitting postures was 66.98% and 165.48% higher than that in rest posture. The above results show that the elastic properties of TLF play an important role in maintaining body static posture and that the forward tilt and sitting postures are likely to induce low back pain (LBP). In conclusion, this study provides preliminary in vivo data for the relationship between body postures and TLF stiffness.