It's probably a ground loop or short caused by different grounds.
The best solution would be to isolate the RPi using a floating power supply.
if i connect a wire between RPI PS and computer it will be ok?
I don't know what is a floating PS
A floating PS means it doesn't have any reference to actual, real ground. V+ is 5V above V-, but both are floating with respect to actual ground.
The computer has a real ground, using the 3rd pin in the power plug. The USB shield is tied to this ground, which means the Picoscope is tied to this ground. That means the ground lead on your Picoscope is actual ground, with a solid connection back to the earth, just like a normal oscilloscope. And just like with a normal oscilloscope, this means you need to be careful what you attach it to. If you attach it to something that's floating you're fine, if you attach it to something that also has its own earth connection, at best you'll create a ground loop, and at worst you'll create a short circuit.
As for how you get a floating supply for your RPi, there are several ways. As rstofer said, a battery would work, or an isolated wall wart (some are, some aren't) would work. You could also use an isolating transformer to isolate the wall wart, or you could use a bench power supply which will almost always have floating outputs.
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