In order to pursue high intrinsic quality factor (Q₀) as well as high accelerating gradient (E_acc) of superconducting radio-frequency niobium cavities, medium temperature baking experiments are carried out on 1.3 GHz single cell cavities at Peking University. The preliminary medium temperature treatments include 350 °C 3 h baking and 400 °C 3 h baking and the 2 K vertical test results turn out to deviate from the expectation, with Q-slope beginning at low field and acceptable but not very high values of Q₀ at high field. Thus, the cavities are treated with 3 updated schemes, which refer to nitrogen doping at 330 °C, optimized medium temperature baking at 300 °C and low temperature baking at 120 °C. These experiment schemes result in considerable values of Q₀ of 2.8-3.4 × 10¹⁰ at medium and high fields and maximum values of E_acc of 27.3-37.9 MV/m. Meanwhile, 5 groups of niobium samples are prepared respectively and treated identically to the cavities. The measurements of TOF-SIMS and XPS demonstrate that compared with baseline sample, the optimized 300 °C baked sample has moderately higher concentrations of carbon and nitrogen and one order lower concentration of hydrogen, similar to the situation of the 330 °C nitrogen doped sample except its lower carbon concentration. It is the lower concentration of hydrogen that yields the high values of Q₀ of the cavities.