How does the secondary coil affect the Q of the primary coil?
This question is really about transformer theory and isn't &quot;made special&quot; because the primary may be part of a tuned circuit.

R_p will be very big and this will be in series to inductor of LC
tank, but if series resistance increase then won't Q will decrease
instead of increasing? What point am I missing?

The point you are missing is that the resistance &quot;projected&quot; from the secondary to the primary is not in series with the primary winding but, in parallel with it.
Imagine a regular (but ideal) transformer that steps-down 240 volts AC to 24 volts AC. Imagine also that the secondary load is a 24 ohm resistor. That would cause a secondary current of 1 amp RMS and, in the primary, will be a current of 0.1 amps RMS.
So, from the perspective of the AC applied to the primary, the load looks like 240 volts ÷ 100 mA = 2400 ohms. That 2400 ohms is 24 ohms x turns ratio squared (10²). In other words, the load is projected from the secondary to appear in parallel with the primary magnified by N².
The higher the step down ratio, the higher the projected load is by N².