Well, with D2 and D5 removed, the circuit now is only Vin and three resistors in series.
What is the current flowing the resistors?
According to Ohm's law, what is the voltage across R6?

 

I USE THE PSPICE TO CALCULATE.
WHY THE PEAK VOLTAGE IS NOT 10V??

 

I USE THE PSPICE TO CALCULATE.
WHY THE PEAK VOLTAGE IS NOT 10V??

The ground should be put at the bottom not there.

 

like this??

 

The peak voltage is still not 10v
why?

 

Why the ground place at the bottom but not on that position??

 

like this??

That seems right except the negative cycle.

The peak voltage is still not 10v
why?

I think that is because your diodes are not ideal. There is some voltage drop across these diodes.

 

Why the ground place at the bottom but not on that position??

Sorry, maybe I am wrong. I am simulating it now.

 

Here is my simulation.

I also changed the ground point and the result doesn't change. Actually, I don't understand the ground symbol here. Maybe, someone like Jony can help us.

 

The ground should be put at the bottom not there.

Not according to the questions in post number 1.

You are measuring the output across R6.

 

Not according to the questions in post number 1.

You are measuring the output across R6.

Thanks. It seems that the voltage across R6 remains unchanged if I move the ground point anywhere around the circuit.

 

It seems that the voltage across R6 remains unchanged if I move the ground point anywhere around the circuit.

I don't think that is a valid statement.

 

I don't think that is a valid statement.

I just simulated all cases and I am right. The voltage across R6 doesn't change at all.
The ground symbol here seems to be needed for simulation purpose not have any effect on the result.
That also makes sense. The voltage across each component should not depend on where we choose ground reference.
However, if we measure voltage at each node in reference to ground, the voltage will different in each case (each case with different ground point).

 

Your correct on your statement. I wasn't thinking clear enough to be posting.