Water level, as an intuitive factor of hydrological conditions, is of great importance for lake management. In this study, periodic structures of water level and its fluctuations in Lake Baiyangdian are analyzed based on wavelet analysis and Seasonal Trend decomposition using Local error sum of squares (STL). The data of monthly time series are devided into three types with an emphasis on anthropogenic influence by water allocation. It is found that intra-annual characteristics of water level fluctuations are the common periodic sturcutures. Water allocation changes the periodic structures by decreasing and weakening the oscillations of water level fluctuations, compared with the faint valid effects of natrual hydrologcial water supplies and short-term climate changes. An irregular water level decline and a short-term oscillation with an irregular periodicity are presented by STL analysis. With seasonality depicted monthly, the influence of water allocation implies irregular oscillatings in high frequency compontent, especially in monthly changes. The water level fluctuations are influenced by seasonal changes demonstrated in three types of time series. The effects of water allocation on seasonality show the differences with continuous single peak oscillations representing no influences and continuous double peak oscillations representing frequent influences. Furthermore, the accumulation of water allocation shows a faint rising trend in average monthly level fluctuations for the last several years. The study is helpful to understand periodic structures and long-term trend changes of water level fluctuations that will facilitate lake management of Lake Baiyangdian.