For the pulser circuit shown below, the output pulse is negetive. If BNC need to be connected to the load, how can output pulse be positive?

 

Use a single stage common emitter circuit with an NPN transistor, similar to the one shown in your circuit.

 

Use transistor or MOSFET ....

 

What if the voltage is 1KV, pulse duration is 1ns?

 

What if the voltage is 1KV, pulse duration is 1ns?

Then i think it will be not easy...!!
or use step transformer ( Fly back)

 

What in the world are you working on that outputs a 1nS -1kV pulse?

I hope you realize that a 1nS pulse will require a circuit with a bandwidth >= 10GHz in order to make a reasonable facsimile of the pulse, but inverted, right? That's in the radar X-band RF spectrum. At those frequencies, a tiny kink in a wire acts like an inductor. It's even difficult to use multilayer ceramic caps due to their parasitic inductance.

In the beginning of your thread, you posted a waveform that's practically identical to the one in this thread:
http://forum.allaboutcircuits.com/showthread.php?t=59546
and the OP is wanting to invert the signal, stretch the pulse, and detect it using a microcontroller.

 

Looks like some kind of charged particle or gamma-ray detector.

 

use an emitter follower op amp.

 

use an emitter follower op amp.

Emitter follower does not invert.

 

The pulser i am working on is not the one as shown in the beginning of the thread. But the structure of the output stage is the same-- a capacitor and a load resistor. The current flows from GND to the load. So that pulse is negative.