  We present timing solutions of PSR B0950+08, using 14 years of observations
at Nanshan 26-m Radio Telescope of Xinjiang Astronomical Observatory. The
braking index of PSR B0950+08 varies from --367 392 to 168 883, which shows an
oscillation with large amplitude ($\sim 10^5$) and uncertainty. Considering the
variation of braking indices and the most probable kinematic age of PSR
B0950+08, a model withe long-term magnetic field decay modulated by short-term
oscillations is proposed to explain the timing data. With this magnetic field
decay model, we discuss the spin and thermal evolution of PSR B0950+08. The
uncertainties of its age are also considered. The results show that
three-component oscillations are the more reasonable for the spin-frequency
derivative distributions of PSR B0950+08, and the initial spin period of PSR
B0950+08 must be shorter than $97\rm\ ms$ when the age is equal to the lower
bound of its kinematic age. The standard cooling model could explain the
surface temperature of PSR B0950+08 with its most probable kinematic age.
Vortex creep heating with a long-term magnetic field decay could maintain a
relatively high temperature at the later stages of evolution and explain the
thermal emission data of old and warm pulsars. Coupling with the long-term
magnetic field decay, an explanation of the temperature of PSR B0950+08 with
roto-chemical heating needs an implausibly short initial rotation period ($P_0
\lesssim 17\rm{ ms}$). The spin and thermal evolution of pulsars should be
studied simultaneously. Future timing, ultraviolet or X-ray observations are
essential for studying the evolution and interior properties of pulsars.
