In light of the effect of the technical solution of the separation joint on the dynamic characteristics of as-built adjacent structures, ambient vibration tests are conducted in two adjacent reinforced concrete frame-shear wall structures separated by an expansion joint. Modes including natural frequencies, damping ratios, and mode shapes are identified. The magnitude-squared coherences are calculated based on the test data obtained from the sensors located on the two sides of the expansion joint simultaneously. Long term monitoring of the vibrations of two adjacent structures is conducted. The trends of natural frequencies of the first three modes varying with the temperature are obtained. The finite element modeling method for the adjacent structures taking into account the expansion joint under different temperatures is proposed. The results show that the dynamic characteristics of the two adjacent structures under ambient vibration are the same due to the continuous floor paving bricks across the expansion joint. The two adjacent structures form a system. The first two natural frequencies almost have no change with the increase of the temperature. The third natural frequency (the one corresponding to the torsional mode due to the continuity of the expansion joint) increases significantly with the increase of the temperature. Slabs with the same width as the separation gap and different elastic moduli are used to simulate the link between the two adjacent structures due to the expansion joint.