Abstract: Magnetars are a type of young and highly magnetized neutron star, which can be broadly categorized into soft γ ray repeaters (SGRs) and anomalous X ray pulsars (AXPs). Their primary energy source comes from their internal ultra-strong magnetic fields. Observations show that magnetars often manifest as X ray sources with significant luminosity variations and, in some cases, as radio and/or optical pulsars. Observational and theoretical studies of magnetars are a major focus in current pulsar research. The braking index of a pulsar is an important physical quantity closely related to its rotational evolution. This paper is divided into two parts. The first part reviews our research on the evolution of the toroidal magnetic field inside strongly magnetized pulsars and applies the theoretical model to the AXP 1E 2259+586. The results indicate that the toroidal magnetic field component is the main driving factor behind transient changes, magnetosphere distortions, and high-energy radiation activities in magnetars. The second part reviews recent advances in the study of pulsar braking indices and derives an expression for the braking index in a vacuum magnetic dipole model that does not depend on the second derivative of the pulsar's rotational period. This expression is applied to five pulsars with known braking indices to verify our model. The proposed expression takes into account variations in magnetic field strength and magnetic inclination angle and includes information about energy loss mechanisms. This new expression is not only applicable to young pulsars without glitches and with low timing noise but also to middle-aged pulsars with small glitches and low timing noise, which is also a major highlight of this paper. The final part summarizes the paper and looks ahead to future research on the magnetic field and rotational evolution of pulsars. We anticipate that through more in-depth X-ray observations, combined with further future studies on magnetar activities, including simultaneous X ray and optical monitoring, we can not only verify our current research findings but also reveal the more complex and rich characteristics of magnetars.