Generally speaking, arachnoid cysts up to 5cm are considered to be more dangerous, but we cannot only look at the size of the tumor, but also need to analyze it together with the specific location and clinical manifestations. Arachnoid cysts are benign, non-tumor cystic space-occupying lesions that may occur in the temporal fossa, anterior base of the skull, etc. Generally speaking, if the tumor size is less than 5 cm, the patient will not experience any obvious abnormal sensations, and the degree of danger is relatively small. Once it exceeds 5 centimeters, patients may experience discomfort such as headache and dizziness, and there is a high risk of rupture, which is dangerous and requires timely treatment. Whether arachnoid cysts are dangerous or not is not only based on the size, but also needs to be combined with the location of the tumor and clinical manifestations of the comprehensive analysis, for example, arachnoid cysts growing in the important functional areas, may be only 3 cm patients will be able to develop significant neurological dysfunction. In conclusion, patients with arachnoid cysts should seek timely medical treatment under the guidance of a doctor to avoid delaying the condition and causing serious consequences.