Hello and welcome to Electronics Point
It's not a square wave; it's a very badly clipped sinewave. My guess is that the transistor is being overdriven and/or is incorrectly biased. Try reducing the amplitude of the V
X and V
Y generators. If you still have problems:
1. What is the frequency, waveform, and amplitude of the V
X voltage generator?
2. What is the frequency, waveform, and amplitude of the V
Y voltage generator?
3. Can you show us the top part of the schematic, including the base bias resistor and the collector load resistor, and the supply voltage.