Question about Stepping motor

When i drive up a stepping motor at lower speed, the vibration is excited than high speed...
Is it a normal condition?
if not, shall i increase the pulse rate from PLC to reduce the vibration?

Yes, it is normal condition.
Try to find,if your stepper motor controller support "Microsteping" and use more microsteps to one full step at lower speeds.
Some controllers supports the possibility to change waveform shaping,too.(For instance sinewave,trapezoid,triangle etc) Try to change it.It may helps.

stepper resonance

Apparently your stepper has entered into a resonance which is not uncommon.
Resonance is an unstable condition when running a step motor. Resonance usually occurs between 2-4 rev/sec and can be alleviated in most situations by putting a load on the motor, changing from full stepping to half or microstepping, or increasing your acceleration through the resonance zone.
Main problem is the loss of torque while in the resonance zone.