Uprising ground-level ozone (O3) and its regional pollution in eastern China are attracting more attention. On top of local precursor emissions and photochemistry, background ozone and long-range transport also contribute significantly to O3 concentrations. To quantify the regional O3 background concentrations and its yearly and seasonal variations, multiple methods, including the principal component analysis (PCA) and the Texas Commission on Environmental Quality (TCEQ) method, were applied for a case study in Shandong (SD) province in North China, where serious O3 pollution occurred frequently yet the background contributions have not been well quantified. Results derived from multiple methods show an overall consistent trend with 2018-2020 averaged regional background O3 (MDA8) of 88.9 μg/m3, accounting for 79.4% of total O3 in the region. From 2018-2020, the changes of regional MDA8 O3 estimated by Methods 1, 2, 3, and 4 are -3.8 μg/m3,1.6 μg/m3, -5.2 μg/m3 and 0.9 μg/m3, respectively. Clear seasonal variations in the regional background O3 are observed, showing a pattern of summer > spring > autumn > winter. In addition, the regional ozone contribution at coastal cities was larger than that for inland cities whereas local O3 formation gradually increased from coastal areas to inland areas. The sea-land wind contribution to O3 in the eastern coastal cities in summer was around 2.1% at the three-year average level, while the local photochemistry to O3 in the inland cities was about 29.7% during ozone season. Local photochemical contribution to O3 in inland cities during ozone pollution episodes can reach up to 55.8%.