have no idea to solve  Go to the Training: Crypto - Digraphs challenge

I understand how the encryption work,but i only can come up with two method from my mind
1.count the frequency e.g in statistic e 12% and hu 10% so e may equal to hu
2.make a program to try all possible combination ,create a number of decrypted text and detect the most common word like"the"or "solution" to filter

Welcome to any suggestion and correction

Don't just think about it in your mind. Analyze the given ciphertext. You might notice a thing or two.

So do you mean i need to analyze the given ciphertext, and guest the letters instead of cracking the key??

No, I just said to analyze the ciphertext. After that, do what makes most sense given the results of your analysis.

i analyzed the ciphertext and find that there are more that 26 bigram in ciphertext space so it contains uppercase and lowercase and punctuation in plaintext space?

AAABAC

I make a simple c++ program to help to analyze the ciphertext and find that the number of bigraph is constant but the number of row(2nd letters) and col(1st letters) is equal and random,any hints T.T

i have an idea like i find the longest word in ciphertext is 16 letters and check the position of repeating letter to guest that word
any suggestion??

My suggestion is to stop asking for hints and suggestions and just try stuff.

That's true.Thanks you XD