public static string ToHexString(string s)
        {
            char[] chars = s.ToCharArray();
            StringBuilder builder = new StringBuilder();
            for (int index = 0; index < chars.Length; index++)
            {
                bool needToEncode = NeedToEncode(chars[index]);
                if (needToEncode)
                {
                    string encodedString = ToHexString(chars[index]);
                    builder.Append(encodedString);
                }
                else
                {
                    builder.Append(chars[index]);
                }
            }
            return builder.ToString();
        }
        /// <summary>
        /// Determines if the character needs to be encoded.
        /// </summary>
        /// <param name="chr"></param>
        /// <returns></returns>
        private static bool NeedToEncode(char chr)
        {
            string reservedChars = "$-_.+!*'(),@=&";
            if (chr > 127)
                return true;
            if (char.IsLetterOrDigit(chr) || reservedChars.IndexOf(chr) >= 0)
                return false;
            return true;
        }
        /// <summary>
        /// Encodes a non-US-ASCII character.
        /// </summary>
        /// <param name="chr"></param>
        /// <returns></returns>
        private static string ToHexString(char chr)
        {
            UTF8Encoding utf8 = new UTF8Encoding();
            byte[] encodedBytes = utf8.GetBytes(chr.ToString());
            StringBuilder builder = new StringBuilder();
            for (int index = 0; index < encodedBytes.Length; index++)
            {
                builder.AppendFormat("%{0}", Convert.ToString(encodedBytes[index], 16));
            }
            return builder.ToString();
        }

//......

string downloadName = ToHexString("中文名称");

//......