直接上代码
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
#header {
position: fixed;
100%;
height: 100px;
top: 0;
left: 0;
background: #ff6444;
filter: alpha(opacity=0);
-moz-opacity: 0;
-khtml-opacity: 0;
opacity: 0;
}
</style>
</head>
<body>
<div id="header"></div>
<div>1</div>
<div>2</div>
<div>3</div>
<div>4</div>
<div>5</div>
<div>6</div>
<div>7</div>
<div>8</div>
<div>9</div>
<div>10</div>
<div>11</div>
<div>12</div>
<div>13</div>
<div>14</div>
<div>15</div>
<div>16</div>
<div>17</div>
<div>18</div>
<div>19</div>
<div>20</div>
<div>21</div>
<div>22</div>
<div>23</div>
<div>24</div>
<div>25</div>
<div>26</div>
<div>27</div>
<div>28</div>
<div>29</div>
<div>30</div>
<div>31</div>
<div>32</div>
<div>33</div>
<div>34</div>
<div>35</div>
<div>36</div>
<div>37</div>
<div>38</div>
<div>39</div>
<div>40</div>
<div>41</div>
<div>42</div>
<div>43</div>
<div>44</div>
<div>45</div>
<div>46</div>
<div>47</div>
<div>48</div>
<div>49</div>
<div>50</div>
<div>51</div>
<div>52</div>
<div>53</div>
<div>54</div>
<div>55</div>
<div>56</div>
<div>57</div>
<div>58</div>
<div>59</div>
<div>60</div>
<div>61</div>
<div>62</div>
<div>63</div>
<div>64</div>
<div>65</div>
<div>66</div>
<div>67</div>
<div>68</div>
<div>69</div>
<div>70</div>
<div>71</div>
<div>72</div>
<div>73</div>
<div>74</div>
<div>75</div>
<div>76</div>
<div>77</div>
<div>78</div>
<div>79</div>
<div>80</div>
<div>81</div>
<div>82</div>
<div>83</div>
<div>84</div>
<div>85</div>
<div>86</div>
<div>87</div>
<div>88</div>
<div>89</div>
<div>90</div>
<div>91</div>
<div>92</div>
<div>93</div>
<div>94</div>
<div>95</div>
<div>96</div>
<div>97</div>
<div>98</div>
<div>99</div>
<div>100</div>
<script type="text/javascript">
var header = document.getElementById('header');
window.onscroll = function() {
var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
var opacity;
// 数字200表示到多少px之后会变成不透明
// 透明度初始为1，随着滚动逐渐为0
opcaity = (1 - (scrollTop / 200) < 0) ? 0 : 1 - (scrollTop / 200);
// 透明度初始为0，随着滚动逐渐为1
opcaity = (scrollTop / 200 > 1) ? 1 : scrollTop / 200;
header.style.opacity = opcaity;
header.style.filter = 'alpha(opacity:'+(opcaity*100)+')';
}
</script>
</body>
</html>