Abstract: The sea level pressure in the East Asia–Northwest Pacific region directly reflects the circulation characteristics of the lower atmosphere, and its dynamical characteristics have considerable effects on the atmospheric circulation situation, the evolution of the pressure system, and the development of weather and climate systems. Therefore, an in-depth analysis of the spatial and temporal evolution characteristics of the sea level pressure field in the East Asia–Northwest Pacific region is of great significance to improve the weather and climate forecasting in China. To investigate the dynamical characteristics of the daily sea level pressure field from the viewpoint of nonlinear dynamics, a new method is used to quantitatively estimate two instantaneous indicators of the sea level pressure attractor: (1) The instantaneous dimension and (2) the instantaneous stability. The instantaneous dimension characterizes the dispersion of the attractor orbit in local space, and the instantaneous stability characterizes the stability of the orbit in local time, which together characterize the instantaneous (daily) dynamical properties of the sea level baroclinic attractor. This paper studies the different spatial and temporal characteristics of the sea level pressure field in the East Asia–Northwest Pacific region by the correspondence between the indicator values of different sizes and the daily sea level pressure circulation field. The main conclusions are as follows: (1) When both indicators of the sea level pressure attractor are low, the spatial characteristics of the corresponding circulation field exhibit a single pressure structure, usually with several strong high- and low-pressure centers facing each other at the east–west direction, while the time characteristics show that the circulation mode can be stable for approximately 10 days. (2) Whereas, when both the indicators are high, the spatial characteristics of the circulation field show the simultaneous existence of multiple weak pressure centers with a chaotic spatial structure. As for the temporal characteristics, the circulation field is extremely unstable and the duration is approximately only one day. (3) In addition, the instantaneous dimension and instantaneous stability were found to have consistent interdecadal trends, both showing a clear downward trend from the 1970s to the 1990s, a rapidly rising trend in the late 1990s, and fluctuating changes after the year 2000.