The urbanization process in the Loess Plateau region, while promoting economic development, also faces the problem that ambient air is seriously polluted. Good urban ventilation can alleviate the contradiction between urban development and livable environment. Therefore, the study focuses on the effect of built-up area expansion on ventilation characteristics for Loess Tableland valley town under the stable background wind. The results simulated by the city-scale computational fluid dynamics (CSCFD) model show that the ventilation performance of urban canopy layer (UCL) with direct interaction between the stable background wind and street canyon airflow is significantly better than that of the UCL with indirect interaction. When the built-up area expands on the valley floor, the expansion is favorable to improve the air quality in the UCL at the background Reynolds number (Reb) less than 5.80 x 105. When the built-up area expands on the top surface of Loess Tableland, the expansion at Reb >= 1.55 x 106 causes a maximum increase of 133.21 % and 64.47 % in the average air age in the upstream UCL and downstream UCL, respectively, which deteriorates the air quality in both the UCLs. This study provides a theoretical reference for future urban planning and optimum location of industrial zones.