The analysis of the nonlinear trends and non-stationary oscillations of high temperatures and heatwaves in Xi'an, based on 8 extreme high temperature indices, is performed using homogenized temperature dataset over the period of 1960-2016. Temporal trends and their impact factors are evaluated by climate diagnosis methods including extremepoint symmetric mode decomposition (ESMD) and correlation analysis.The effect of air-sea anomalies such as El Nino and West Pacific Subtropical High (WPSH) to extreme high temperatures are explored. The results are as follows: (1) Using the in-homogeneity dataset, the warming trends of warm nights, summer days, hot nights, and heatwaves are underes-timated, the downtrend in the heating energy consumption and the uptrend in the cooling energy consumption are overestimated; while the trend change of warm days and high temperature days are affected little by in-homogeneity data. (2) Meanwhile, warm days, heatwaves, high temperature days and cooling degree days, reflecting the characteristics of extreme high temperatures in the daytime, show the fluctuations of decrease-increase-decrease-increase; warm nights and hot nights, which describe the features of extreme high temperatures in the nighttime, exhibit the low stationary fluctuation before 1993 and high stable fluctuation afterwards; (3) With regards to influencing factors, sea surface temperature anomaly in the Nino 4 region is closely related to high temperatures in Xi'an. When sea surface temperature in the Nino 4 region is high abnormally, warm days, summer days and cooling energy consumption increase significantly whereas heating energy consumption decreases obviously. When the intensity of WPSH increases and its control area enlarges, with its ridge point expanding westward, warm nights, summer days and hot days increase obviously, while heating energy consumption decreases.