I to P converter convert 4-20mA signal to 3-15 psi signal then, this 3-15psi signal goes to actuator to actuate the control valve position. But my question is this small amount of pressure(3-15psi) is not enough for moving the stem of Control valve. so how it's work. Please Explain.

 

With the right sized actuator, a 3-15psi control manages to move the valve stem on numerous HVAC valves or light duty industrial valves. It does work, if the actuator/spring combination are sized correctly.

For heavy duty industrial valves, 3-15psi is also used as the control signal to drive a pneumatic positioner that uses air supply pressure greater than the control signal's 15psi to drive the actuator/valve stem, used in Zone 0 or Class 1, Div 1 areas. I find that 4-20mA is more commonly used as a control signal to a positioner than 3-15psi nowadays.