I am using Debian as guest in vBox and it was work without problem. but when I install another machine (another debian) in my vBox, my first Debian dose not boot and it is freezing in black screen. when I enter to my first debian OS in recovery mode I can make ls to my directories but when trying to run it I got on the black screen. How can fix the problem please?

Welcome to LinuxQuestions.
I have 3 different versions of debian as guests as well as sometimes Ubuntu, Mint and others. I have never had problem like you described.

What version of VirtualBox are you running?

Are you trying to run multiple guests at the same time?

How much memory total is installed as well as how much memory are you assigning to each guest? Are you using multiple processors for your guests?

I assume you are creating a completely new guest with a different virtual disk drive? Are you using different VM names as well as hostnames for each guest?

I increase the cpi from 1 to 2.
increase the video memory to 128 MB.
removed the new installation machine,*
and the same problem.

What is the size of the base memory?
The display should be set to VMSVGA
Can you boot from the live ISO file?