  Pulsar's position, proper motion and parallax are important parameters in
timing equations. It is a really challenging work to fit astrometric parameters
accurately through pulsar timing, especially for pulsars that show irregular
timing properties. As the fast development of related techniques, it is
possible to measure astrometric parameters of more and more pulsars in a
model$\textrm{-}$independent manner with the Very Long Baseline Interferometry
(VLBI). In this work, we select 16 normal pulsars, whose parallax and proper
motion have not been successfully fitted with timing observations or show
obvious differences with corresponding latest VLBI solutions, and do further
studies on their timing properties. After updating astrometric parameters in
pulsar ephemerides with the latest VLBI measurements, we derive the latest
rotation solutions of these pulsars with observation data at S and
C$\textrm{-}$band obtained from the Shanghai Tian Ma Radio Telescope (TMRT).
Compared with spin frequency $\nu$ inferred from previous rotation solutions,
the newly$\textrm{-}$fitted $\nu$ show differences larger than 10$^{-9}$ Hz for
most pulsars. The contribution of the Shklovsky effect to period derivative
$\dot{P}$ can be properly removed taking advantages of accurate proper motion
and distance of target pulsars measured by VLBI astrometry. This further leads
to a precise estimate of intrinsic characteristic age $\tau_{\rm c}$.
Differences between the newly$\textrm{-}$measured $\tau_{\rm c}$ and
corresponding previous results are as large as 2% for some pulsars. VLBI
astrometric parameter solutions also lead to better measurements of timing
irregularities. For PSR B0154$+$61, the glitch epoch (MJD 58279.5) measured
with previous ephemeris is about 13 d later than the result (MJD 58266.4)
obtained with VLBI astrometric parameter solutions.
