Thank for your protocol first.However, the protocol couldn't...

First of all you mentioned your standard curve has also not changed color. That would mean that the DMMB solution would not have been prepared correctly.
Second, please read through the notes. Using samples with high salt and albumin content may affect the assay; I am not sure what mouse model you are using to collect your urine samples. Sometimes diluting samples 1:10 can help, especially if you believe the concentration of sulfated GAGs in your samples will fall above the standard curve. You could also add a gel filtration step before the DMMB assay.
Also, in the notes we mention that DMMB requires the length of glycosaminoglycan chain be over a tetrasaccharide. GAGs present in the urine are significantly smaller than those present in tissues, so please make sure your samples are appropriate for this assay.