Inverter with 50 Hz

hi! can someone help me how to decrease the value of frequency ?

high frequency

http://www.electronics-tutorials.ws/waveforms/555_oscillator.html
See section on 50% duty cycle astable in above link. Your capacitor value is way too small.
As a power inverter this circuit has many problems, there are better ways of doing power transfer, you haven't loaded your transformer secondary with a resistor value that represents the power that you require on the secondary side. Do that and you will find out what I mean. Also, Spice is not helpful if you don't model components like transformers more accurately, Spice is just a tool, you still have to do all of the engineering for it to make some sense.
Cheers,
Richard

okay Richard. thanks for your concern