I run ngspice simulation inside xschem with

tran 100u 300u

, theoretically the spice simulation must be completed in 3 step, but when i run simulation I se more and more steps... why?

The transient analysis time step is calculated by ngspice such that the approximation of the numerical solution of differential equations is acceptable. The user supplied

100u

is a 'suggested timestep' but the required timestep is much lower, expecially if there are signals that transition from low to high or the other way around in nanoseconds. The timestep used in calculations by ngspice is variable. Your user supplied value is very high so in this case ngspice uses its own estimated (variable) timestep.