Abstract: To investigate the characteristics of CCN (cloud condensation nuclei) and aerosol particle size distribution, along with their influencing factors in the Middle Tianshan region of Xinjiang, data on aerosol and CCN number concentration were collected from 4 to 25 August 2019. Measurements were conducted using an aerosol wide-range particle size spectrometer and a CCN counter, complemented by meteorological data and the HYSPLIT model for comprehensive analysis. The results revealed that during the observation period, the average aerosol number concentration in Tianshan was 3607±4105 cm⁻³, with Aitken mode particles accounting for the highest proportion (64.76%). The mean CCN number concentrations at supersaturation levels of 0.1%, 0.2%, 0.4%, 0.6%, and 0.8% were 185±185 cm⁻³, 648±345 cm⁻³, 1576±1045 cm⁻³, 2077±1417 cm⁻³, and 2408±1657 cm⁻³, respectively. The average aerosol number concentration exhibited a unimodal distribution, with the peak particle size located at 29.5 nm. Diurnal variations in aerosol number concentration showed peaks at 0800 BJT (Beijing time), 1700 BJT, and 2000 BJT, influenced primarily by the boundary layer height diurnal variation, mountain-valley winds, emission sources, and other factors. During summer, aerosol particles in the Tianshan Mountains exhibited distinct distribution modes. The Aitken mode particles exhibited a bimodal distribution, while the accumulation mode particles displayed a trimodal distribution. The number concentration of nucleation mode particles showed an unimodal diurnal variation, likely attributable to NPF (new particle formation). The CCN activation spectrum fitting yielded N=3111S^0.896 (where N is the CCN number concentration at supersaturation S), characteristic of a clean continental type. The diurnal variation of CCN number concentration peaked at 0800 BJT, 1800 BJT, and 2100 BJT. NPF was found to enhance the CCN number concentration in the Tianshan Mountains. At supersaturation levels of 0.1%, 0.2%, 0.4%, 0.6%, and 0.8%, the average CCN concentrations on NPF days were respectively 13%, 18%, 25%, 22%, and 20% higher than those on non-NPF days. RH (relative humidity) had a more significant impact on the concentration of nucleation mode particles compared with other particle modes. As RH increased, the aerosol size distribution shifted toward larger particle sizes. When RH exceeded 60%, the number of concentrations of both aerosols and CCN decreased with increasing RH. Additionally, aerosol and CCN number concentrations were higher under northerly and easterly winds, influenced by the region’s topography and valley wind patterns. The Tianshan region experienced four main types of air masses: western (29.2%), northwestern (29.3%), northeastern (27.0%), and southern (14.5%). The northeastern air mass originating from the northern Tianshan Mountains exhibited the highest aerosol number concentration at 5449±3793 cm⁻³, while the southern air mass had the lowest at 1971±2087 cm⁻³. These differences are attributed to variations in air mass direction, movement paths, and the influence of the northern urban zone. At all supersaturation levels, the northeastern air mass also showed higher CCN number concentrations (which ranged from 100% to 150%) than the southern air mass.