How can I simulate the circuit with floating node in Orcad simulation?
Assuming you have some connection to Spice ground on the phototransistor side and you're only getting it complaining about the base connection, you can connect the base to the emitter via a very high value resistor (eg. 1G). You can't have parts of your circuit (such as the secondary of a transformer) just floating around. 

When this pin is used, it's typically connected to the emitter by a much lower value resistor (perhaps 100K), which decreases the sensitivity but increases the speed.