Hi, guys!
I have read a lot algorithms about random-generation,most of them use the system time as the seed,and then calculate it with
formula Rand_Number = (Rand_Seed * X + Y) mod Z, why use the system time as the seed,is it random enough? then if the seed is random,why calculate it again through this formula?

It depends.

For security-related applications, where you would want to create random keys so that a third party couldn't guess them, using only the time is not enough. The operating system typically provides means of creating better random values (Linux and Mac have /dev/random, Windows has a cryptography API). The OS uses seeds such as disk writes per second, processor cache misses, port I/O, time, etc.

For artificial intelligence, fuzzy logic or things like that, using 1-2 seeds is fine. But because time is very predictable and changes at a fixed rate, then in some cases it might not seem random enough. Then you would use a formula like you described to constantly recalculate the seed number based on the current time and the previous seed number.

Time is a good seed, because it is one of the few metrics that are readily available for all programs at any permission levels. You would need admin access to get values from other system hardware. Another value that is often used in games is the time between keypresses, however many other applications don't get any keyboard input at all.

The formula Rand_Number = (Rand_Seed * X + Y) mod Z is good, because there is a good chance the number will overflow because of the multiplication. This means the higher digits are discarded, because they won't fit into the 32 or 64 bits the computer uses to store the number. Without the formula, the seed number would only change linearly, because time is linear.

If I use a 8-bit MCU,there is no explicit system time,so how can i
get the seed source?!Thank you!

I don't know what hardware you have or what the purpose is, but there are a few generic options.

The last used seed number could be stored into permanent memory (if available) and used the next time when needed. This way the random values are good as long as the seed isn't lost.

Maybe you don't even need very good random values? That would be the case if the values are needed for doing something not so important, like making a visual effect, for example. Then you could just use any preset seed value.

If you have some input to your MCU, then you can sample values from it. For example, consider an electronic dice. One way to get a random value is to measure the time between pressing the key and releasing it. (You may not have time, but maybe you have a timer). Be creative!

There are special additional chips for generating random bits, but you can also use things like accelerometers, thermistors, photoresistors, etc. to gather variable data from the environment.