In order to analyze reference crop evapotranspiration (ET0) and its spatio-temporal variation of complexity in Liaoning Province and based on the daily meteorological data of 56 weather stations in Liaoning Province from 1954 to 2006, daily ET0 was calculated by the Penman-Montieth formula and its complexity was calculated by the Lempel-Ziv algorithm. The results showed that: The average daily ET0 was within the range of (2.380 ± 0.300) mm ,the maximum value 2.690mm achieved in 1965 and the minimum value 2.098mm achieved in 1954; average annual ET0 was in a decreasing trend and the trend was at a rate of -0.017mm • 10a; the cumulative average value of all decades is negative except the 1960s,Liaoning Province spatial distribution of mean annual ET0 rendered from the northwest to the southeast coast inland decreasing trend , and in addition to Zhangwu stations, Fuxin station , Fengcheng stations, Zhuanghe stations and King County station showed a significant upward trend , other sites mostly downward trend and 1/ 3 of the above sites showed a significant downward trend ; an increasing trend was observed from northwest to southeast coastal areas in 50a ET0 complexity with higher complexity the southeast coastal areas and lower complexity northeast inland areas. First increase and then decrease trend were found in multi-year average yearly ET0 complexity. In last 20 year, statistically significant decreasing trend was observed generally in Liaoning province. ET0 complexity in liaoning province were significantly positive correlation with the temperature and wind speed, and relative humidity has significant positive correlation.